IB/hns: checking for IS_ERR() instead of NULL

The hns_roce_v1_create_lp_qp() returns NULL on error, not error pointers.

Fixes: bfcc681b ("IB/hns: Fix the bug when free mr")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>