We need to align the RX buffer size to at least 16 byte so that IP
doesn't mis-behave. This is required by HW.

Changes from v2:
- Align UP and not DOWN (David)

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>