hwmon: (g760a) Make rpm_from_cnt static

Function rpm_from_cnt is only used internally so it can be made
static. Make it inline while we're here, for performance reasons
(although hopefully gcc would figure out by itself...)

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Herbert Valerio Riedel <hvr@gnu.org>
Acked-by: Martin Michlmayr <tbm@cyrius.com>