phy: qcom-ufs: Remove common layer phy exit callback

The common layer phy exit callback ufs_qcom_phy_exit()
calls phy_power_off() that has no meaning when phy_power_off()
callback is already registered with the phy provider and
the consumer makes use of the same.
Instead, add a no-op specific phy_exit() callback for now
to add the exit sequence at a later point.

Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>