sfc: Remove unnecessary dependencies on I2C

Only the SFC4000 code, now moved to sfc-falcon, needed I2C.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>