hw/net/allwinner-sun8i-emac.c: Fix REG_ADDR_HIGH/LOW reads

hw/net/allwinner-sun8i-emac.c: Fix REG_ADDR_HIGH/LOW reads

Coverity points out (CID 1421926) that the read code for
REG_ADDR_HIGH reads off the end of the buffer, because it does a
32-bit read from byte 4 of a 6-byte buffer.

The code also has an endianness issue for both REG_ADDR_HIGH and
REG_ADDR_LOW, because it will do the wrong thing on a big-endian
host.

Rewrite the read code to use ldl_le_p() and lduw_le_p() to fix this;
the write code is not incorrect, but for consistency we make it use
stl_le_p() and stw_le_p().

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>