hwmon: (adt7470) Replace power-of-two test

Since <linux/log2.h> already supplies a power-of-two test, there's no
point in having this source file redefine it again.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>