CAIF: Fix IPv6 support in receive path for GPRS/3G

Checks version field of IP in the receive path for GPRS/3G data
and appropriately sets the value of skb->protocol.

Signed-off-by: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>