arcnet: cleanup sizeof parameter

[ Upstream commit 087d273caf4f7d3f2159256f255f1f432bc84a5b ]

This patch doesn't change the compiled code because ARC_HDR_SIZE is 4
and sizeof(int) is 4, but the intent was to use the header size and not
the sizeof the header size.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>