ARCNET: fix hard_header_len limit

ARCNET: fix hard_header_len limit

For arcnet the bare minimum header only contains the 4 bytes to
specify source, dest and offset (1, 1 and 2 bytes respectively).
The corresponding struct is struct arc_hardware.

The struct archdr contains additionally a union of possible soft
headers. When doing $insertusecasehere packets might well
include short (or even no?) soft headers.

For this reason only use arc_hardware instead of archdr to
determine the hard_header_len for an arcnet device.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>