hwmon: (lm90) Move 16-bit value read to a separate function

Move the code which aggregates two 8-bit register values into a 16-bit
value to a separate function. We'll need to do it a second time soon and
I don't want to duplicate the code.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Martyn Welch <martyn.welch@gefanuc.com>