dpaa2-eth: properly handle buffer size restrictions

Depending on the WRIOP version, the buffer size on the RX path must by a
multiple of 64 or 256. Handle this restriction properly by aligning down
the buffer size to the necessary value. Also, use the new buffer size
dynamically computed instead of the compile time one.

Fixes: 27c87486

 ("dpaa2-eth: Use a single page per Rx buffer")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>