sfc: get drvinfo driver name from outside the common code

Since ethtool_common.o will be built into both sfc and sfc_ef100 drivers,
 it can't use KBUILD_MODNAME directly.  Instead, make it reference a
 string provided by the individual driver code.

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