iw_cxgb4: check return value of alloc_skb

Function alloc_skb() will return a NULL pointer when there is no enough
memory. However, the return value of alloc_skb() is directly used
without validation in function send_fw_pass_open_req(). This patches
checks the return value of alloc_skb() against NULL.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>