Use regmap for local caching, to hide register read/write address
differences, and for multi-byte operations. With this change,
the driver specific lock is no longer necessary.

While at it, check errors seen when initializing the chip and bail out
if chip initialization fails.
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>