scsi: ufs: core: Reduce the clock scaling latency

Wait at most 20 ms before rechecking the doorbells instead of waiting for a
potentially long time between doorbell checks.

Reviewed-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>