[SCSI] usb-storage: do not increment cmd->serial_number

The isd200 sub-driver increments the command serial number despite not
using it at all in it's routine for sending internal scsi commands.
Remove the increment to prepare for removing the serial_number field.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>