sfc: support Medford2 frequency adjustment format

Support increased precision frequency adjustment format (FP44) used
 by Medford2 adapters.

Signed-off-by: Laurence Evans <levans@solarflare.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>