pinctrl: mvebu: remove unused variable

A cleanup caused a harmless warning:

drivers/pinctrl/mvebu/pinctrl-kirkwood.c: In function 'kirkwood_pinctrl_probe':
drivers/pinctrl/mvebu/pinctrl-kirkwood.c:460:19: error: unused variable 'res' [-Werror=unused-variable]

The obvious fix is to remove the declaration of the now unused
variable.

Fixes: ad9ec4ec ("pinctrl: mvebu: switch drivers to generic simple mmio")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>