hwmon: (amc6821) sign extension temperature

hwmon: (amc6821) sign extension temperature

commit 4538bfbf2d9f1fc48c07ac0cc0ee58716fe7fe96 upstream.

Converts the unsigned temperature values from the i2c read
to be sign extended as defined in the datasheet so that
negative temperatures are properly read.

Fixes: 28e6274d8fa67 ("hwmon: (amc6821) Avoid forward declaration")
Signed-off-by: Jared Bents <jared.bents@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
[groeck: Dropped unnecessary continuation line]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>