hwmon: (ad7314) Fix build warning

The following build warning is seen in some configurations.

drivers/hwmon/ad7314.c: In function 'ad7314_show_temperature':
drivers/hwmon/ad7314.c:70: warning: 'data' may be used uninitialized in this function

Fix by overloading the return value from ad7314_spi_read with both data and
error code (the returned data is really u16 and needs to be converted into a
signed value anyway).

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Jean Delvare <khali@linux-fr.org>