scsi: ufs: unexport descritpor reading functions

Unexport ufshcd_read_device_desc and ufshcd_read_string_desc there is no
really possibility to calling them directly outside of UFS context.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>