stmmac: replace open coded __netdev_alloc_skb_ip_align() with actual call

This also matches with the sibling call netdev_alloc_skb_ip_align() made in
rx fast path.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>