hwmon: (w83627ehf) Fix negative 8-bit temperature values

Since 8-bit temperature values are now handled in 16-bit struct
members, values have to be cast to s8 for negative temperatures to be
properly handled. This is broken since kernel version 2.6.39
(commit bce26c58.)

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: stable@kernel.org	# 2.6.39+
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>