gpio: mcp23s08: Fix clearing of interrupt.

The mcp23s18 is configurable on clearing the interrupt on either reading
INTCAP or GPIO. Since driver reads INTCAP in IRQ and not the GPIO reg need
to set control byte for this mode.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>