[SCSI] stex: fix scan of nonexistent lun

During a manual scan, a user can send command to a nonexistent
lun, precisely at the point of max_lun. Normally it's possible
(but not required) that the firmware has the knowledge that it
is an invalid lun. In the particular case when max_lun is 256,
however, the nonexistent lun 256 will be confused with lun 0,
because the lun member in a request message is only u8, and 256
will become 0. So we need to fix the problem, at least, at the
driver level.

Signed-off-by: Ed Lin <ed.lin@promise.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>