Fix incorrect usage of NET_IP_ALIGN

The driver used NET_IP_ALIGN to remove some additional padding inside of
the rx_fixup function. On many architectures NET_IP_ALIGN defaults to 2
which removed the correct amount of bytes.

On MCORE2-machines commit ea812ca1b06113597adcd8e70c0f84a413d97544
introduces a change which sets NET_IP_ALIGN to 0 by default. Which
triggered the bug on these machines.

This fix introduces a new RXW_PADDING define and uses this instead of
NET_IP_ALIGN. The name was taken from the original SMSC7500 driver which
is provided by SMSC.

Signed-off-by: Nico Erfurth <ne@erfurth.eu>
Tested-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>