staging: comedi: usbdux: use common code path to check the cmd->scan_begin_arg

There are currently separate code paths to check the scan_begin_arg based
of the USB speed. Refactor the code to use a common code path and simplify
the code.

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>