sfc: Fix dependency for SFC_MTD

Randy Dunlap found that SFC_MTD was selected when sfc was built-in and
the MTD core was a module.  Don't allow that combination.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>