staging: comedi: add facility to directly query subdevice timing constraints

Adds facility to directly query the hardware speed limits of subdevices,
in particular for scan_begin and convert signals.  This information can be
critical for integrating comedi with other hardware modules, and also
comedi modules together with software where software requires specific
timing capabilities in order to properly coordinate multiple devices.

Currently, comedi_command_test almost satisfies this need, but really only
for when *_src == TRIG_TIMER.  For *_src == TRIG_EXT, comedi_command_test
does not help at all.  For many subdevices, one might simply use
*_src==TRIG_TIMER in command_test in order to determine these limits.  For
other subdevices, this tactic does not work since *_src == TRIG_TIMER might
not be valid.  There is also the possibility that the timing limits are
different between the TRIG_TIMER and TRIG_EXT modes.

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>