wimax: 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.

Due to the removal of duplication in the netlink policies,
this reduces the size of wimax by almost 1k.

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