staging: comedi: addi_apci_2032: only disable triggered interrupts

The interrupt handler disables all interrupt sources when a valid
interrupt occurs.  Just disable the triggered interrupt source so we can
still get interrupts for the other interrupt source.

Also add a comment indicating why the triggered interrupt source is
disabled.  The interrupt sources are level-sensitive and indicate
hardware errors that are likely to be persistent, so if we reenabled
them they would just keep triggering repeatedly.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>