[SCSI] enclosure: fix error path - actually return ERR_PTR() on error

we also need to clean up and free the cdev.

Reported-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>