driver: net: ldpaa: Fix Rx buffer alignment

MC 0.7.1.2 enforces limitation i.e.: "Packets may be corrupted
in several combinations of buffer size and frame offsets.
Workaround: Use buffers that are of size that is a multiple of 256, and
frame offset that is a multiple of 256"

Updating the DPNI Eth driver to comply with the restriction.

Signed-off-by: Bogdan Hamciuc <bogdan.hamciuc@freescale.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>