hwmon: (adt7475) replace find_nearest() with find_closest()

The adt7475 has had find_nearest() since it's creation in 2009. Since
then find_closest() has been introduced and several drivers have been
updated to use it. Update the adt7475 to use find_closest() and remove
the now unused find_nearest().

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>