ieee802154: use genl_register_family_with_ops()

This simplifies the code since there's no longer a need to
have error handling in the registration.

Unfortunately it means more extern function declarations are
needed, but the overall goal would seem to justify this.

While at it, also fix the registration error path - if the
family registration failed then it shouldn't be unregistered.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>