ASoC: si476x: Fix locking of core

The conversion of the si476x to regmap removed locking of the core during
register updates, allowing things like power state changes for the MFD to
happen during a register update. Avoid this by taking the core lock in the
DAI operations (which are the only things that do register updates) as we
used to do in the open coded register I/O functions.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>