staging: comedi: adl_pci9118: exttrg source is always EXTTRG_AI

The 'source'  passed to pci9118_exttrg_{add,del}() is always EXTTRG_AI.
Remove the parameter and unnecessary sanity checking. Also, since there
is only one vaild exttrg source, remove the unnecessary 'exttrg_users'
member from the private data.

The pci9118_exttrg_{add,del}() functions always succeed. 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>