sfc: get timer configuration from adapter

On SFN8000 series adapters the MC provides a method to get the timer
quantum and the maximum timer setting. We revert to the old values if the
new call is unavailable.

Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>