net/usb: Misc. fixes for the LG-VL600 LTE USB modem

Add checking for valid magic values (needed for stability in the event
corrupted packets are received) and remove some other unneeded checks.
Also, fix flagging device as WWAN (Bugzilla bug #39952).

Signed-off-by: Mark Kamichoff <prox@prolixium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>