mctp: Avoid warning if unregister notifies twice

Previously if an unregister notify handler ran twice (waiting for
netdev to be released) it would print a warning in mctp_unregister()
every subsequent time the unregister notify occured.

Instead we only need to worry about the case where a mctp_ptr is
set on an unknown device type.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>