gpio: Drop parent irq assignment during cascade setup

We want to set the irq parent for interrupts that we're setting up to be
cascaded from another interrupt controller, but we may or may not have
already mapped the gpiochip irqs into the kernel's virtual irq number
space at this point. If we have mapped irqs before calling here, then
we've gone through gpiochip_irq_map() and called irq_set_parent()
already. If we haven't mapped irqs, then the gpiochip is dynamically
mapping irqs and we can rely on gpiochip_irq_map() or the gpio driver's
irqdomain ops to setup the irq parent properly.

Either way, setting the parent here when cascading the gpiochip doesn't
make much sense because it should be done at irq mapping time. In the
dynamic mapping case, this code is mapping virq 0 to some parent virq in
a loop. While that's benign, let's drop this code to simplify.

Cc: Evan Green <evgreen@chromium.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>