[media] media.h: mark alsa struct in media_entity_desc as TODO

[media] media.h: mark alsa struct in media_entity_desc as TODO

The alsa struct in struct media_entity_desc is now marked as deprecated.
However, the alsa struct should remain as it is since it cannot be replaced
by a simple major/minor device node description. The alsa struct was designed
to be used as an alsa card description so V4L2 drivers could use this to expose
the alsa card that they create to carry the captured audio. Such a card is not
just a PCM device, but also needs to contain the alsa subdevice information,
and it may map to multiple devices, e.g. a PCM and a mixer device, such as the
au0828 usb stick creates.

This is exactly as intended and this cannot and should not be replaced by a
simple major/minor.

However, whether this information is in the right form for an ALSA device such
that it can handle udev renaming rules as well is another matter. So mark this
alsa struct as TODO and document the problems involved.

Updated the documentation as well to reflect this and to add the 'major'
and 'minor' field documentation.

Updated the documentation to clearly state that struct dev is to be used for
(sub-)devices that create a single device node. Other devices need their own
structure here.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>