cxgb4: add missing release on skb in uld_send()

In the implementation of uld_send(), the skb is consumed on all
execution paths except one. Release skb when returning NET_XMIT_DROP.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>