staging: comedi: addi_apci_3120: tidy up apci3120_reset()

This function is only called during the attach of the driver and it's
called after the private data has been kzalloc'd. There is no reason
to clear any of the private data members.

Simplify the function by just writing '0' to the necessary registers
to disable all the interrupt sources and stop the timers.

For aesthetics, change the return type to void.

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