hwmon: (sht3x) set initial jiffies to last_update

Handling the wraparound requires the data->last_update to be set to an
initial jiffies value. Otherwise on 32-bit systems you will not be able
to request a reading till the 5 minute jiffies rollover happens.

Cc: Guenter Roeck <linux@roeck-us.net>
Cc: David Frey <david.frey@sensirion.com>
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Fixes: 7c84f7f8

 ("hwmon: add support for Sensirion SHT3x sensors")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>