RDMA/cxgb4: Fix crash when peer address is 0.0.0.0

When using rping -c -a 0.0.0.0 with iw_cxgb4, the system crashes when
rdma_connect() is called.  ip_dev_find() will return NULL, but pdev is
accessed anyway.

Checking that pdev is NULL and returning -ENODEV prevents the system
from crashing.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>