staging: comedi: ni_mio_common: split out ao arming from ni_ao_inttrig

AO device arming was previously done as a part of ni_ao_inttrig which is
called as a result of the user calling comedi_internal_trigger.  For
start_src == TRIG_EXT, this does not make very much sense since external
triggers should not conceptually need to be software triggered also.  This
patch splits out the arming functionality to allow arming to specifically
and separately be done via the CONFIG_INSN_ARM ioctl command.

In order to provide backwards compatibility, this patch also provides
automatic arming if ni_ao_inttrig is simply called.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>