RDMA/bnxt_re: Fix an IS_ERR() vs NULL check

The bnxt_re_mmap_entry_insert() function returns NULL, not error pointers.
Update the check for errors accordingly.

Fixes: 360da60d

 ("RDMA/bnxt_re: Enable low latency push")
Link: https://lore.kernel.org/r/8d92e85f-626b-4eca-8501-ca7024cfc0ee@moroto.mountain
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>