Phonet: include generic link-layer header size in MAX_PHONET_HEADER

This fixes an OOPS in hard_header if a Phonet address is assigned to a
non-Phonet network interface.

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