gpio: tqmx86: really make IRQ optional

The tqmx86 MFD driver was passing IRQ 0 for "no IRQ" in the past. This
causes warnings with newer kernels.

Prepare the gpio-tqmx86 driver for the fixed MFD driver by handling a
missing IRQ properly.

Fixes: b868db94

 ("gpio: tqmx86: Add GPIO from for this IO controller")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>