pinctrl-sx150x: Use handle_bad_irq instead of handle_edge_irq

Althought the function passed as a "handler" during GPIO chip
instantiation is not going to ever be called, specifying handle_edge_irq
there makes for a rather confusing read, both because no "ack" callback
in specified for irqchip and because there's no acking action is
necessary.

Specify handle_bad_irq instead a make a note of the situation. This
commit should be a no-op behaviour wise.

Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>