pinctrl: microchip-sgpio: don't do RMW for interrupt ack register

The interrupt ack register has the usual "write one to clear" semantics.
No read-modify-write is required here.

This is also a preparation patch to change the sgpio_clrsetbits() to use
regmap_update_bits() which don't write the value if it is not changed.

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220226204507.2511633-3-michael@walle.cc


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>