ASoC: davinci: vcif must be a module if SND_DAVINCI_SOC is

It is possible to configure a kernel with SND_DAVINCI_SOC=m and
SND_DM365_VOICE_CODEC=y, which results in a link error:

sound/built-in.o: In function `davinci_vcif_probe':
sound/soc/davinci/davinci-vcif.c:223: undefined reference to `davinci_soc_platform_register'

The best way to avoid this is to make SND_DM365_VOICE_CODEC
a tristate option that depends on SND_DAVINCI_SOC, so it
can only be a module or disabled when the base driver is
a loadable module

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>