Fix to handle hard_header_len update

When ifconfig up/down sequence is initiated hard_header_len
get updated incrementally for each ifconfig up /down sequence,
this leads invalid hard_header_len, moving to lan78xx_bind
to have one time update of hard_header_len addresses the issue.
Signed-off-by: Nisar Sayed <Nisar.Sayed@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>