sky2: fix limited auto negotiation

The sky2 driver would always try all possible supported speeds even
if the user only asked for a limited set of speed/duplex combinations.

Reported-by: Mohsen Hariri <m.hariri@gmail.com>
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>