ASoC: soc-cache: Warn on syncing any non-writable registers

When syncing the cache, if the driver has given us a writable_register()
callback, use it to check if we are syncing a non-writable register
and if so warn the user.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>