scsi: core: Clean up struct ufs_saved_pwr_info

The "is_valid" field of the struct ufs_saved_pwr_info is no longer used,
and this struct can be replaced by struct ufs_pa_layer_attr without any
changes to the functionality.

Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Link: https://lore.kernel.org/r/20230330012918.13748-1-stanley.chu@mediatek.com
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>