devices: fix af_packet GSO check

Type: fix

This fixes the GSO size calculation in af_packet
which didn't include the ethernet size.
This is not ideal, as we default to the host
side mtu to check whether a packet is GSO or not
and to set the GSO size. But there doesn't seem
to be more info passed with the packets.

Change-Id: I9769e1dd21d5989b4cf67295352b5535454f88d1
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>