wlcore: avoid using NET_IP_ALIGN for RX alignment

NET_IP_ALIGN can be overriden on different architectures
and therefore cannot be used in the RX path to account
for the 2 bytes added for alignment (either by the FW
in the case of 18xx or by the host for 12xx).
Instead use an internal define.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>