gpio/langwell: Simplify demux loop

Use __ffs() to find the pending interrupt source instead of looping 32
times.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Alek Du <alek.du@intel.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>