sfc: use IS_ENABLED() checks for CONFIG_SFC_SRIOV

One local variable has become unused after a recent change:

drivers/net/ethernet/sfc/ef100_nic.c: In function 'ef100_probe_netdev_pf':
drivers/net/ethernet/sfc/ef100_nic.c:1155:21: error: unused variable 'net_dev' [-Werror=unused-variable]
  struct net_device *net_dev = efx->net_dev;
                     ^~~~~~~

The variable is still used in an #ifdef. Replace the #ifdef with
an if(IS_ENABLED()) check that lets the compiler see where it is
used, rather than adding another #ifdef.

This also fixes an uninitialized return value in ef100_probe_netdev_pf()
that gcc did not spot.

Fixes: 7e056e23 ("sfc: obtain device mac address based on firmware handle for ef100")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>