staging: comedi: addi_apci_3120: move apci3120_exttrig_enable() to driver source

Move this helper function from the included source file into the main driver
source file.

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>