Staging: comedi: dt9812: remove dt9812.h

An extra .h file is not needed for the dt9812 driver so
merge it into the .c file.

Cc: Anders Blomdell <anders.blomdell@control.lth.se>
Cc: David Schleef <ds@schleef.org>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>