regmap: Don't attempt block writes when syncing cache on single_rw devices

If the device can't support block writes then don't attempt to use raw
syncing which will automatically generate block writes for adjacent
registers, use the existing _single() block syncing implementation.

Reported-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org