RDMA/cxgb4: set error code on kmalloc() failure

If kmalloc() fails in c4iw_alloc_ucontext(), the function
leaves but does not set an error code in ret variable:
it will return 0 to the caller.

This patch set ret to -ENOMEM in such case.

Cc: Steve Wise <swise@opengridcomputing.com>
Cc: Steve Wise <swise@chelsio.com>
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>