staging: comedi: addi_apci_3120: tidy up apci3120_exttrig_{enable,disable}()

Merge these two functions and use an 'enable' parameter to determine if the
external trigger needs to be enabled or disabled.

This function always succeeds and the callers don't check the return. 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>