pinctrl: mcp23s08: Do not complain about unsupported params

It is expected that some of these operations won't work on each and
every HW. Previously, even a simple `cat
/sys/kernel/debug/pinctrl/spi1.1/pinconf-pins` caused excessive dmesg
output.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Phil Reid <preid@electromag.com.au>
Cc: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>