bnx2: Advertise nothing when speed is forced

The current code does not reset the advertisement register when the speed
is forced, leaving the default advertisement value of 10 Mbps.  This does
not work with some link partners when the next patch enables auto-mdix.

Set advertisement register to 0 if the speed is forced.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>