gpio: stmpe: Fix forgotten refactoring

We actually handle the gpio_irq_chip set-up properly
now despite what the comment says. Also assign this
pointer along with the rest of the gpio_irq_chip
setup code.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201019134429.65563-1-linus.walleij@linaro.org