cbus: retu: IRQ demux optimization

it's definitely not always that we will have
all 16 interrupts fired at the same time, so
in order to avoid looping so many times, we
are now using ffs() which is implemented
(on ARM) using the far better clz instruction.

This will save us quite some loops and could
improve IRQ latency on Retu significantly
(no actual measurements were made, though)

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>