pinctrl: oxnas: remove set but not used variable 'arg'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/pinctrl/pinctrl-oxnas.c: In function oxnas_ox810se_pinconf_set:
drivers/pinctrl/pinctrl-oxnas.c:905:6: warning: variable arg set but not used [-Wunused-but-set-variable]
drivers/pinctrl/pinctrl-oxnas.c: In function oxnas_ox820_pinconf_set:
drivers/pinctrl/pinctrl-oxnas.c:944:6: warning: variable arg set but not used [-Wunused-but-set-variable]

It is never used since commit 4b0c0c25

 ("pinctrl:
oxnas: Add support for OX820"), so can be removed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20190725142419.29892-1-yuehaibing@huawei.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>