Fixes the following sparse warning:

drivers/ata/ahci_mvebu.c:85:5: warning:
 symbol 'ahci_mvebu_stop_engine' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>