Staging: ipack/bridges/tpci200: Clean up interrupt handling.

Previously the return value from tpci200_interrupt was not quite correct
if a slot had caused an interrupt but no handler was instellalled:
IRQ_NONE was returned.  However in this case we react to the interrupt
by disabling the IPack device interrupt.

Basically there are two cases the code now distinguishes:
 - The tpci200 has raised an interrupt.  We handle it and return
   IRQ_HANDLED.
 - Our device did not raise an interrupt. We return IRQ_NONE.

Signed-off-by: Jens Taprogge <jens.taprogge@taprogge.org>
Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>