net/usb: Add IPv6 support to the LG-VL600 LTE USB modem driver

The LG-VL600 LTE USB modem supports IPv6, but uses and expects an IPv4
ethertype (0x800) for these packets instead of the standard 0x86dd.
This patch peeks at the IP version in the L3 header and sets the
ethertype appropriately for IPv6 packets.

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