gpio: fix 'CONFIG_GPIO_IRQCHIP' comments

These two typos were introduced in commit 14250520 ("gpio: add IRQ
chip helpers in gpiolib").

The correct symbol name is CONFIG_GPIOLIB_IRQCHIP.

[jkosina@suse.cz: add changelog]
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>