hw/net: Avoid casting non-const pointer, use address_space_write()

hw/net: Avoid casting non-const pointer, use address_space_write()

The NetReceive prototype gets a const buffer:

  typedef ssize_t (NetReceive)(NetClientState *, const uint8_t *, size_t);

We already have the address_space_write() method to write a const
buffer to an address space. Use it to avoid:

  hw/net/i82596.c: In function ‘i82596_receive’:
  hw/net/i82596.c:644:54: error: passing argument 4 of ‘address_space_rw’ discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]

This commit was produced with the included Coccinelle script
scripts/coccinelle/exec_rw_const.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>