via-velocity: forced 1000 Mbps mode support.

Full duplex only. Half duplex 1000 Mbps is not supported.

Signed-off-by: David Lv <DavidLv@viatech.com.cn>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Seguier Regis <rseguier@e-teleport.net>
Signed-off-by: David S. Miller <davem@davemloft.net>