gpio: pca953x: Survive spurious interrupts

gpio: pca953x: Survive spurious interrupts

The pca953x driver never checks the result of irq_find_mapping(),
which returns 0 when no mapping is found. When a spurious interrupt
is delivered (which can happen under obscure circumstances), the
kernel explodes as it still tries to handle the error code as
a real interrupt.

Handle this particular case and warn on spurious interrupts.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201005140217.f816f14-1-maz@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>