ASoC: pxa: change ac97 dependencies

Enabling both the old AC97_BUS code and the new AC97_BUS_COMPAT causes
problems because both modules provide an exported snd_ac97_reset()
function.

I had tried to fix the problem of having both coexist earlier, but
my patch only prevented them from being built-in. This is because
of a special Kconfig feature that lets a symbol have a dependency
on another one being disabled, but still allow both to be loadable
modules.

Changing the dependency to =n avoids that problem, now we can only
build the new driver if the old one is completely disabled.

If we could figure out a way to let rename one of the reset
functions and have each driver link to exactly the old or
the compat code, that would also work, but I could not find if
that's possible.

Fixes: bec5ecdf ("ASoC: pxa: avoid AC97_BUS build warning")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>