scsi: xen-scsifront: Call scsi_done() directly

Conditional statements are faster than indirect calls. Hence call
scsi_done() directly.

Link: https://lore.kernel.org/r/20211007204618.2196847-6-bvanassche@acm.org


Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>