staging: comedi: me4000: absorb ai_write_timer()

This function is only called by me4000_ai_do_cmd(). Absorb it.

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>