[SCSI] mptsas: don't complain on bogus slave_alloc calls

When people use the userspace scanning facilities on SAS hardware the
LLDD gets bogus slave_alloc calls.  Just fail those gracefully instead
of printing a warning in mptsas and another one in the midlayer.

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