ata: ahci_mvebu: ahci_mvebu_stop_engine() can be static

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>

drivers/ata/ahci_mvebu.c

@@ -82,7 +82,7 @@ static void ahci_mvebu_regret_option(struct ahci_host_priv *hpriv)
  *
  * Return: 0 on success; Error code otherwise.
  */
-int ahci_mvebu_stop_engine(struct ata_port *ap)
+static int ahci_mvebu_stop_engine(struct ata_port *ap)
 {
 	void __iomem *port_mmio = ahci_port_base(ap);
 	u32 tmp, port_fbs;