Using local caching in this driver had few benefits. It used cached values
for two seconds and then re-read all registers from the chip even if the
user only accessed a single attribute. On top of that, alarm attributes
were stale for up to four seconds (the first status register read reports
and clears an alarm, the second reports it cleared). Use regmap instead
for caching. Do not re-read non-volatile registers, and do not cache
volatile registers.

As part of this change, handle register read and write address differences
in regmap code. This is necessary to avoid problems with caching in the
regmap core, and ultimately simplifies the code.

Also, errors observed when reading from and writing to registers are no
longer ignored.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>