w1: fix slave driver registration error message

W1 core prints "Failed to register master driver" if error happens
on registering SLAVE driver. Fix it.

Signed-off-by: Dmitry Artamonow <mad_soft@inbox.ru>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>