IB/rxe: Make rxe_counter_name static

rxe_counter_name is used in rxe_hw_counters.c only. Make it static.

Fixes: 0b1e5b99 ('IB/rxe: Add port protocol stats')
Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Reviewed-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>