net: iosm: create default link via WWAN core

Utilize the just introduced WWAN core feature to create a default netdev
for the default data (IP MUX) channel.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
CC: M Chetan Kumar <m.chetan.kumar@intel.com>
CC: Intel Corporation <linuxwwan@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>