pinctrl: rockchip: split out verification of mux settings

We need to avoid calling regmap functions from irq handlers, so the next
commit is going to move the call to rockchip_set_mux() into an
irq_bus_sync_unlock handler.  But we can't return an error from there so
we still need to check the settings from rockchip_irq_set_type() and we
will use this new rockchip_verify_mux() function from there.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>