ASoC: codecs: wcd938x: fix wcd module dependency

With SND_SOC_ALL_CODECS=y and SND_SOC_WCD938X_SDW=m, there is a link
error from a reverse dependency, since the built-in codec driver calls
into the modular soundwire back-end:

x86_64-linux-ld: sound/soc/codecs/wcd938x.o: in function `wcd938x_codec_free':
wcd938x.c:(.text+0x2c0): undefined reference to `wcd938x_sdw_free'
x86_64-linux-ld: sound/soc/codecs/wcd938x.o: in function `wcd938x_codec_hw_params':
wcd938x.c:(.text+0x2f6): undefined reference to `wcd938x_sdw_hw_params'
x86_64-linux-ld: sound/soc/codecs/wcd938x.o: in function `wcd938x_codec_set_sdw_stream':
wcd938x.c:(.text+0x332): undefined reference to `wcd938x_sdw_set_sdw_stream'
x86_64-linux-ld: sound/soc/codecs/wcd938x.o: in function `wcd938x_tx_swr_ctrl':
wcd938x.c:(.text+0x23de): undefined reference to `wcd938x_swr_get_current_bank'
x86_64-linux-ld: sound/soc/codecs/wcd938x.o: in function `wcd938x_bind':
wcd938x.c:(.text+0x2579): undefined reference to `wcd938x_sdw_device_get'
x86_64-linux-ld: wcd938x.c:(.text+0x25a1): undefined reference to `wcd938x_sdw_device_get'
x86_64-linux-ld: wcd938x.c:(.text+0x262a): undefined reference to `__devm_regmap_init_sdw'

Work around this using two small hacks: An added Kconfig dependency
prevents the main driver from being built-in when soundwire support
itself is a loadable module to allow calling devm_regmap_init_sdw(),
and a Makefile trick links the wcd938x-sdw backend as built-in
if needed to solve the dependency between the two modules.

Fixes: 04544222

 ("ASoC: codecs: wcd938x: add audio routing and Kconfig")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210721150510.1837221-1-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>