hw/net/i82596: Correct command bitmask (CID 1419392)

hw/net/i82596: Correct command bitmask (CID 1419392)

The command is 32-bit, but we are loading the 16 upper bits with
the 'get_uint16(s->scb + 2)' call.

Once shifted by 16, the command bits match the status bits:

- Command
  Bit 31 ACK-CX   Acknowledges that the CU completed an Action Command.
  Bit 30 ACK-FR   Acknowledges that the RU received a frame.
  Bit 29 ACK-CNA  Acknowledges that the Command Unit became not active.
  Bit 28 ACK-RNR  Acknowledges that the Receive Unit became not ready.

- Status
  Bit 15 CX       The CU finished executing a command with its I(interrupt) bit set.
  Bit 14 FR       The RU finished receiving a frame.
  Bit 13 CNA      The Command Unit left the Active state.
  Bit 12 RNR      The Receive Unit left the Ready state.

Add the SCB_COMMAND_ACK_MASK definition to simplify the code.

This fixes Coverity 1419392 (CONSTANT_EXPRESSION_RESULT):

  /hw/net/i82596.c: 352 in examine_scb()
  346         cuc = (command >> 8) & 0x7;
  347         ruc = (command >> 4) & 0x7;
  348         DBG(printf("MAIN COMMAND %04x  cuc %02x ruc %02x\n", command, cuc, ruc));
  349         /* and clear the scb command word */
  350         set_uint16(s->scb + 2, 0);
  351
  >>>     CID 1419392:    (CONSTANT_EXPRESSION_RESULT)
  >>>     "command & (2147483648UL /* 1UL << 31 */)" is always 0 regardless of the values of its operands. This occurs as the logical operand of "if".
  352         if (command & BIT(31))      /* ACK-CX */
  353             s->scb_status &= ~SCB_STATUS_CX;
  >>>     CID 1419392:    (CONSTANT_EXPRESSION_RESULT)
  >>>     "command & (1073741824UL /* 1UL << 30 */)" is always 0 regardless of the values of its operands. This occurs as the logical operand of "if".
  354         if (command & BIT(30))      /*ACK-FR */
  355             s->scb_status &= ~SCB_STATUS_FR;
  >>>     CID 1419392:    (CONSTANT_EXPRESSION_RESULT)
  >>>     "command & (536870912UL /* 1UL << 29 */)" is always 0 regardless of the values of its operands. This occurs as the logical operand of "if".
  356         if (command & BIT(29))      /*ACK-CNA */
  357             s->scb_status &= ~SCB_STATUS_CNA;
  >>>     CID 1419392:    (CONSTANT_EXPRESSION_RESULT)
  >>>     "command & (268435456UL /* 1UL << 28 */)" is always 0 regardless of the values of its operands. This occurs as the logical operand of "if".
  358         if (command & BIT(28))      /*ACK-RNR */
  359             s->scb_status &= ~SCB_STATUS_RNR;

Fixes: Covertiy CID 1419392 (commit 376b851909)
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>