staging: comedi: addi_apci_1564: clean up apci1564_interrupt()

Remove the checks for interrupts from unknown sources.  This situation
should never occur and the checks were doing nothing to help the
situation.

Also, the portion of the function for handling counter interrupts is
reapeated four times (once for each counter), but is completely identical
save for the register is is accessing, so we can handle all four counters
with a for loop.

Finally, the interrupt handler is incorrectly setting and then checking
devpriv->timer_select_mode before processing some of the triggered
interrupts, so just remove all occurrences of this.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk.>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>