scsi: ufs: ufs-mediatek: Fix error checking in ufs_mtk_init_va09_pwr_ctrl()

The function regulator_get() returns an error pointer. Use IS_ERR() to
validate the return value.

Link: https://lore.kernel.org/r/20211222070930.9449-1-linmq006@gmail.com
Fixes: cf137b3ea49a ("scsi: ufs-mediatek: Support VA09 regulator operations")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
