net: smc91x: Remove an unused variable

Commit cb6e0b36 ("net: smc91x: make use of 4th parameter to devm_gpiod_get_index")
caused the following build warning:

drivers/net/ethernet/smsc/smc91x.c:2208:6: warning: unused variable 'res' [-Wunused-variable]

Remove the unused 'res' variable.

Reported-by: Olof's autobuilder <build@lixom.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>