[SCSI] Alter the scsi_add_device() API to conform to what users expect

The original API returned either an ERR_PTR() or a refcounted sdev.
Unfortunately, if it's successful, you need to do a scsi_device_put() on
the sdev otherwise the refcounting is wrong.

Everyone seems to expect that scsi_add_device() should be callable
without doing the ref put, so alter the API so it is (we still have
__scsi_add_device with the original behaviour).

The only actual caller that needs altering is the one in firewire ...
not because it gets this right, but because it acts on the error if one
is returned.

Acked-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>