sky2: add support for receive hashing

Sky2 hardware supports hardware receive hash calculation.
Now that Receive Packet Steering is available, add support
to enable it.

This version does not depend on CONFIG_RPS. Also set_flags rejects
all values except RXHASH, so driver won't have to change next time
somebody adds a new one.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>