net: wwan: Add WWAN port type attribute

The port type is by default part of the WWAN port device name.
However device name can not be considered as a 'stable' API and
may be subject to change in the future. This change adds a proper
device attribute that can be used to determine the WWAN protocol/
type.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>