tg3: Fix temperature reporting

The temperature registers appear to report values in degrees Celsius
while the hwmon API mandates values to be exposed in millidegrees
Celsius. Do the conversion so that the values reported by "sensors"
are correct.

Fixes: aed93e0b ("tg3: Add hwmon support for temperature")
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Prashant Sreedharan <prashant@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Cc: stable@vger.kernel.org [v3.6+]
Signed-off-by: David S. Miller <davem@davemloft.net>