ASoC: rt5514: work around link error

The main rt5514 driver optionally calls into the SPI back-end to load
the firmware. This causes a link error when one driver selects rt5514
as built-in and another driver selects rt5514-spi as a loadable module:

sound/soc/codecs/rt5514.o: In function `rt5514_dsp_voice_wake_up_put':
rt5514.c:(.text+0xac8): undefined reference to `rt5514_spi_burst_write'

As a workaround, this adds another silent symbol, to force rt5514-spi
to be built-in for that configuration. I'm not overly happy with
that solution, but couldn't come up with anything better. Using
'IS_REACHABLE()' would break the case that relies on the loadable
module, and all other ideas would result in more complexity.

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