IB/mlx5: Remove dead code from alloc_cached_mr()

The only place that assigns mr inside the loop already does a break.
So "if (mr)" will never be true here since the function initializes mr
to NULL at the top.  We can just drop the extra if and break here.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Acked-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>