fibocom: Fix GTRNDIS parsing function
This fixes a bug introduced in a2a0e2d7. Since the state pointer is always non-NULL, the function will always fail if GTRNDIS is inactive, i.e. the modem will return state=0 and no cid field.
Signed-off-by: Sven Schwermer sven.schwermer@disruptive-technologies.com