scsi: ufs: use async operation for hibern8 operation

When enabling/disabling hibern8, we will call flush_work
to wait for the task finished. Using async operation
to avoid long waiting time.

Bug: 138085490
Test: Boot, cat /sys/kernel/debug/ufshcd0/show_hba and
check hibern8_exit_cnt
Signed-off-by: Martin Liu <liumartin@google.com>
Change-Id: I427cd8e40cbc6674a145142babbd92a1062dc511