scsi: ufs: revamp string descriptor reading

Define new a type: uc_string_id for easier string handling and less
casting. Reduce number or string copies in price of a dynamic allocation.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Tested-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>