comedi_board_info: reset n_chans_for_generic_timed for each subdevice

comedi_board_info: reset n_chans_for_generic_timed for each subdevice

Add variable `def_n_chans_for_generic_timed` to hold the preferred
number of channels to use in calls to `comedi_get_cmd_generic_timed()`.
Reset the `n_chans_for_generic_timed` variable to this value each time
around the loop for each subdevice before clipping the value to the
number of channels the subdevice has.