hwmon: (lm90) Rename temperature conversion functions to match usage

The encoding of temperatures varies between chips and modes.  So do not
use "temp1" or "temp2" in the names of the conversion functions, but
specify the encoding.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Martyn Welch <martyn.welch@gefanuc.com>