scsi: ufs-mediatek: Make ufs_mtk_wait_link_state static

scsi: ufs-mediatek: Make ufs_mtk_wait_link_state static

Fix build warning reported by kernel test robot:

Warning:
>> drivers/scsi/ufs/ufs-mediatek.c:181:5: warning: no previous prototype
>> for 'ufs_mtk_wait_link_state' [-Wmissing-prototypes]

Link: https://lore.kernel.org/r/20200616095120.14570-1-stanley.chu@mediatek.com
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>