uas: Fix / cleanup usb_uas_task error handling

-The correct error if we cannot find the dev is INCORRECT_LUN rather then
 INVALID_INFO_UNIT
-Move the device not found check to the top so we only need to do it once
-Remove the dev->lun != lun checks, dev is returned by scsi_device_find
 which searches by lun, so this will never trigger

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
