Add missing description of the spg argument of ata_msense_control().

Fixes: df60f9c6 ("scsi: ata: libata: Add ATA feature control sub-page translation")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230523074701.293502-1-dlemoal@kernel.orgReviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>