usb: storage: Use the correct CBW lengths

USB UFI uses fixed 12-byte commands (as does RBC, which is not
supported), but SCSI does not have this limitation. Use the correct
command block lengths depending on the subclass.

Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Marek Vasut <marex@denx.de>