staging: comedi: remove DPRINTK macro

All users of this macro have been converted to use dev_dbg(). Remove
the unused macro.

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>