wwan: core: relocate ops registering code

It is unlikely that RTNL callbacks will call WWAN ops (un-)register
functions, but it is highly likely that the ops (un-)register functions
will use RTNL link create/destroy handlers. So move the WWAN network
interface ops (un-)register functions below the RTNL callbacks to be
able to call them without forward declarations.

No functional changes, just code relocation.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>