gpio: bcm_kona: rename confusing variables

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. Rename the
"gpio" variable to "hwirq" to reflect what it is. Rename one
instance of "virq" to "child_irq" that better describes what
it is.

Cc: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>