[media] demux.h: Fix a few documentation issues

demux.h was lacking documentation for the DMX_FE_ENTRY macro:
	./drivers/media/dvb-core/demux.h:511: WARNING: c:func reference target not found: DMX_FE_ENTRY

While here, get rid of unused parameters and fix a few minor issues
at the header file.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>