Phonet: allocate separate ARP type for GPRS over a Phonet pipe

A separate xmit lock class supports GPRS over a Phonet pipe over a TUN
device (type ARPHRD_NONE).

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>