staging: comedi: adl_pci9118: merge pci9118_exttrg_{add,del}()

For aesthetics, merge these two helper functions and add a parameter,
'enable', to determine if the external trigger is being added (enabled)
or deleted (disabled).

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>