pinctrl: mcp23s08: remove unused variables from pinconf_set

Variable mask and val are not used in the mcp_pinconf_set().

Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>