hwmon: (adt7410) Don't re-read non-volatile registers

Currently each time the temperature register is read the driver also reads the
threshold and hysteresis registers. This increases the amount of I2C traffic and
time needed to read the temperature by a factor of ~5. Neither the threshold nor
the hysteresis change on their own, so once we have read them, we should be able
to just use the cached value of the registers. This patch modifies the code
accordingly and only reads the threshold and hysteresis registers once during
probe.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>