RDMA/mlx5: Properly check return value of mlx5_get_uars_page

Starting from commit 72f36be0 ("net/mlx5: Fix mlx5_get_uars_page to
return error code") the mlx5_get_uars_page() call returns error in case
of failure, but it was mistakenly overlooked in the merge commit.

Fixes: e7996a9a ("Merge tag v4.15 of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git"

)
Reported-by: Alaa Hleihel <alaa@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>