ALSA: deprecate MIPS AU1X00 AC97 driver

Now that an ASoC variant is available, tell users that this
driver is now living on borrowed time...

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>