In case an under-voltage happens before probing the driver wont
write the critical warning into the kernel log. So don't init
of last_throttled during probe and fix this issue.

Fixes: 74d1e007 ("hwmon: Add support for RPi voltage sensor")
Reported-by: "Noralf Trønnes" <noralf@tronnes.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>