staging: comedi: das16: absorb das16_ai_enable()

This function is only called by das16_cmd_exec(). Absorb it to clarify 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>