hwmon: (lm90) Refactor reading of config2 register

Several vendors implement a second configuration register, which we
check during device detection. Refactor the code to avoid duplication.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Stijn Devriendt <sdevrien@cisco.com>
Cc: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>