regmap: cache: Don't attempt to sync non-writeable registers

In the regcache_default_sync, if a register isn't writeable, then
_regmap_write will return an error and the rest of the sync will be
aborted.  Avoid this by checking if a register is writeable before
trying to sync it.

Signed-off-by: Dylan Reid <dgreid@chromium.org>
Signed-off-by: Mark Brown <broonie@linaro.org>