drm/i2c: tda998x: report whether we actually handled the IRQ

Rather than always reporting that the interrupt was handled, we should
report whether we did handle the interrupt.  Arrange to report IRQ_NONE
for cases where we found nothing to do.

This allows us to (eventually) recover from stuck-IRQ problems, rather
than causing the kernel to solidly lock up.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>