scsi: ufs: Move the ufs_is_valid_unit_desc_lun() definition

Move the definition of this function from a public into a private header
file since it is only used inside the UFS core.

Link: https://lore.kernel.org/r/20220419225811.4127248-29-bvanassche@acm.org


Tested-by: Bean Huo <beanhuo@micron.com>
Reviewed-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>