sfc: use ALIGN macro for aligning frame sizes

Don't open-code it.

CC: Solarflare linux maintainers <linux-net-drivers@solarflare.com>
CC: Shradha Shah <sshah@solarflare.com>
CC: netdev@vger.kernel.org
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>