staging: comedi: dt282x: remove redundant comedi_check_trigger_arg_min()

The 'convert_arg' sets the acquisition timing of the analog input command.
The maximum speed (the minimum timing) depends on the board 'ai_speed' which
if always >= 4000. Remove the redundant 'min' check,

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>