sis190: fix gigabit negotiation

Fixes an initialization error; the chip negotiates gigabit, but
the driver mistakenly handled it as 100Mb.
Changes based on both SiS own GPL driver and forcedeth.
Hopefully should fix
	http://bugzilla.kernel.org/show_bug.cgi?id=9735
	http://bugzilla.kernel.org/show_bug.cgi?id=11149

Signed-off-by: Riccardo Ghetta <birrachiara@tin.it>
Signed-off-by: David S. Miller <davem@davemloft.net>