wm831x_power: Fix off-by-one at free_irq()

An error handling in wm831x_power_probe() mistakenly frees a failed-to-
request irq as well as other irqs. I added missing decrement of the loop
counter.

Signed-off-by: Takeshi Yoshimura <yos@sslab.ics.keio.ac.jp>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>