mfd: ab8500-debugfs: Keep count of IRQs in debugfs

This patch adds a counter to the sysfs file dynamically created
by debugfs. It also fixes an array index error.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Reviewed-by: Mattias NILSSON <mattias.i.nilsson@stericsson.com>