dpaa_eth: fix the RX headroom size alignment

The headroom reserved for received frames needs to be aligned to an
RX specific value. There is currently a discrepancy between the values
used in the Ethernet driver and the values passed to the FMan.
Coincidentally, the resulting aligned values are identical.

Fixes: 3c68b8ff

 ("dpaa_eth: FMan erratum A050385 workaround")
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>