staging: comedi: das16: absorb das1600_mode_detect() into caller

This function is only called during the board attach to determine the
pacer clock speed for the boards capable of bursting.

Absorb it into the caller and remove the attach noise.

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>