net/ethernet: xgmac don't set .driver twice

Cleanup the .driver setup to just do it once, to avoid
the following sparse warning:

drivers/net/ethernet/calxeda/xgmac.c:1914:10: warning: Initializer entry defined twice
drivers/net/ethernet/calxeda/xgmac.c:1920:10:   also defined here

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>