RDMA/nes: checking for NULL instead of IS_ERR

nes_reg_phys_mr() returns ERR_PTRs on error.  It doesn't return NULL.

This bug has been there for a while, but we recently changed from
calling a function pointer to calling nes_reg_phys_mr() directly so now
Smatch is able to detect the bug.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>