net: wwan: Fix WWAN config symbols

There is not strong reason to have both WWAN and WWAN_CORE symbols,
Let's build the WWAN core framework when WWAN is selected, in the
same way as for other subsystems.

This fixes issue with mhi_net selecting WWAN_CORE without WWAN and
reported by kernel test robot:

Kconfig warnings: (for reference only)
   WARNING: unmet direct dependencies detected for WWAN_CORE
   Depends on NETDEVICES && WWAN
   Selected by
   - MHI_NET && NETDEVICES && NET_CORE && MHI_BUS

Fixes: 9a44c1cc

 ("net: Add a WWAN subsystem")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>