gpio: em: drop references to "virtual" IRQ

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: Magnus Damm <damm@opensource.se>
Reviewed-by: Ian Molton <ian.molton@codethink.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>