RDMA/bnxt_re: checking for NULL instead of IS_ERR()

bnxt_re_alloc_mw() doesn't return NULL, it returns error pointers.

Fixes: 9152e0b7 ("RDMA/bnxt_re: HW workarounds for handling specific conditions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>