mfd: ab8500-debugfs: Remove the racy fiddling with irq_desc

First of all drivers have absolutely no business to dig into the internals
of an irq descriptor. That's core code and subject to change. All of this
information is readily available to /proc/interrupts in a safe and race
free way.

Remove the inspection code which is a blatant violation of subsystem
boundaries and racy against concurrent modifications of the interrupt
descriptor.

Print the irq line instead so the information can be looked up in a sane
way in /proc/interrupts.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201210194044.157283633@linutronix.de