modemmanager: add missing ppp ifname option value

modemmanager: add missing ppp ifname option value

The pppd supports the option to configure the name of the network interface
at startup using the ifname option.

The pppd can overwrite the name of the network interface at startup using
the 'ifname' configuration option. If this option is not specified, the new
network interface is created according to the following scheme:
ppp<number>, e.g. ppp0, ppp1 and so on. This can therefore change with
every start if the number has been already assigned to another ppp connection.

In order to always get the same interface name with the modemmanger proto via
ppp, it is created according to the new scheme by setting the 'ifname':
ppp-<uci network interface name>, e.g. ppp-wwan.

Signed-off-by: Florian Eckert <[email protected]>