scsi: ufs: fix static checker errors in ufshcd_system_suspend

This patch fixes newly introduced sparse warning in
ufshcd_system_suspend, introduced by UFS power management series.

Sparse warning:
drivers/scsi/ufs/ufshcd.c:5118 ufshcd_system_suspend()
error: we previously assumed 'hba' could be null (see line 5089)

To fix it, we return 0 in case HBA is not initialized or is
not powered.

Signed-off-by: Dolev Raviv <draviv@codeaurora.org>
Reviewed-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>