gpio: pcf857x: only use set_irq_flags() on ARM

As per the pattern from other GPIO drivers, use set_irq_flags()
on ARM only, use irq_set_noprobe() on other archs.

Also rename the argument "virq" to just "irq", this IRQ isn't
any more "virtual" than any other Linux IRQ number, we use
"hwirq" for the actual hw-numbers, "virq" is just bogus.

Cc: George Cherian <george.cherian@ti.com>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>