scsi: ufs: add a low-level __ufshcd_issue_tm_cmd helper

Add a helper that takes a utp_task_req_desc and issues it, which will
be useful for UFS bsg support.  Rewrite ufshcd_issue_tm_cmd0x to use
this new helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>