net/tap: fix protocol field for non-IP

When non IP packets are sent on TUN interface, the logic put Ipv6 as
protocol field in header. With the current patch, the check is modified
for ipv4, ipv6 and non ip.

Fixes: 204d026a3922 ("net/tap: support tun")

Suggested-by: Ophir Munk <ophirmu@mellanox.com>
Signed-off-by: Vipin Varghese <vipin.varghese@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>