pinctrl: mcp23s08: simplify i2c pdata handling

Simplify i2c pdata handling, so that it uses pdata when available
and falls back to reading device properties otherwise.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>