power: supply: max77705_charger: rework interrupts

Current implementation uses handle_post_irq to actually handle chgin
irq. This is not how things are meant to work in regmap-irq.

Remove handle_post_irq, and request a threaded interrupt for chgin.

Fixes: a6a494c8 ("power: supply: max77705: Add charger driver for Maxim 77705")
Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>