mfd: arizona: Check errors from regcache_sync()

If the control bus is unrelabile we may hit errors during regcache_sync(),
especially given that it tends to be one the most dense bursts of I/O in
many systems.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>