RDMA/cxgb4: Fix memory leaks in c4iw_alloc() error paths

c4iw_alloc() bails out without freeing the storage that 'devp' points to.

Picked up by Coverity - CID 1204241.

Fixes: fa658a98 ("RDMA/cxgb4: Use the BAR2/WC path for kernel QPs and T5 devices")
Signed-off-by: Christoph Jaeger <christophjaeger@linux.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>