comgt: ncm: allow specification of interface name

comgt: ncm: allow specification of interface name

Add ifname property to UCI, which can be used to override the
autodetected interface name in case the detection fails due to having
none or more than one interface exposed by the modem, which is not
explicitly linked to TTY port. This is needed on certain variants of ZTE
MF286R built-in modem, which exposes both RNDIS and CDC-ECM interfaces
on the modem, on which the automatic detection may select the wrong
network interface.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>