gpio: adnp: rename "virq" to "child_irq"

This variable is confusingly named, the different Linux IRQs
aren't any more virtual than any other Linux IRQ. Give it a
non-misleading name.

Acked-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>