net: stmmac: 16KB buffer must be 16 byte aligned

The 16KB RX Buffer must also be 16 byte aligned. Fix it.

Fixes: 7ac6653a ("stmmac: Move the STMicroelectronics driver")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>