scsi: ufs: core: Remove unnecessary wmb() after ringing doorbell

Currently, the doorbell is written to and a wmb() is used to commit it
immediately.

wmb() ensures that the write completes before following writes occur, but
completion doesn't mean that it isn't stored in a buffer somewhere.  The
recommendation for ensuring this bit has taken effect on the device is to
perform a read back to force it to make it all the way to the device. This
is documented in device-io.rst and a talk by Will Deacon on this can be
seen over here:

    https://youtu.be/i6DayghhA8Q?si=MiyxB5cKJXSaoc01&t=1678

But, completion and taking effect aren't necessary to guarantee here.

There's already other examples of the doorbell being rung that don't do
this. The writel() of the doorbell guarantees prior writes by this thread
(to the request being setup for example) complete prior to the ringing of
the doorbell, and the following wait_for_completion_io_timeout() doesn't
require any special memory barriers either.

With that in mind, just remove the wmb() altogether here.

Fixes: ad1a1b9c ("scsi: ufs: commit descriptors before setting the doorbell")
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Link: https://lore.kernel.org/r/20240329-ufs-reset-ensure-effect-before-delay-v5-10-181252004586@redhat.comReviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/ufs/core/ufshcd.c

@@ -7090,10 +7090,7 @@ static int __ufshcd_issue_tm_cmd(struct ufs_hba *hba,
 
 	/* send command to the controller */
 	__set_bit(task_tag, &hba->outstanding_tasks);
-
 	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TASK_REQ_DOOR_BELL);
-	/* Make sure that doorbell is committed immediately */
-	wmb();
 
 	spin_unlock_irqrestore(host->host_lock, flags);