af_packet: use sizeof instead of constant in spkt_device

This small patch removes access to the last element of the spkt_device
array through a constant. Instead, it is accessed by sizeof() to respect
possible changes in if_packet.h.

Signed-off-by: Daniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>