pinctrl: mcp23s08: fix interrupt handling regression

interrupt handling was broken with conversion to using regmap caching.
cached_gpio value was updated by boolean status instead of gpio reading.

Fixes: 8f38910b ("pinctrl: mcp23s08: switch to regmap caching")
Tested-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Dmitry Mastykin <mastichi@gmail.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>