staging: comedi: comedi_test: implement INSN_CONFIG_GET_CMD_TIMING_CONSTRAINTS

Adds implementation of the new INSN_CONFIG_GET_CMD_TIMING_CONSTRAINTS
instruction.

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>