[SCSI] usb: protocol - convert to accessors and !use_sg code path removal

 - Use scsi data accessors and remove of !use_sg code path

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Acked-by: Matthew Dharm <mdharm-scsi@one-eyed-alien.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>