scsi: Drop useless null test in scsi_unit_attention()

req was created by scsi_req_alloc(), which initializes req->dev to a
value it dereferences.  req->dev isn't changed anywhere else.
Therefore, req->dev can't be null.

Drop the useless null test; it spooks Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>