scsi: ufs: skip shutdown if hba is not powered

In some cases, hba may go through shutdown flow without successful
initialization and then make system hang.

For example, if ufshcd_change_power_mode() gets error and leads to
ufshcd_hba_exit() to release resources of the host, future shutdown flow
may hang the system since the host register will be accessed in unpowered
state.

To solve this issue, simply add checking to skip shutdown for above kind of
situation.

Link: https://lore.kernel.org/r/1568780438-28753-1-git-send-email-stanley.chu@mediatek.com
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Acked-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>