regmap: Report if we actually handled an interrupt in regmap-irq

While the IRQ core doesn't currently support shared threaded interrupts
that's no reason for drivers not to do their bit and report IRQ_NONE when
they don't get an interrupt. This allows the core spurious/wedget interrupt
detection support to do its thing.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>