Staging: comedi: make comedi_reset_async_buf local to comedi core

No one outside of the comedi core calls this function, so don't export
it to the world.

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>