hwmon: (ad7314) Adds missing spi_dev initialization

This driver was recently moved from IIO (where it worked) to hwmon (where
it doesn't.) This breakage occured because the hwmon version neglected to
correctly initialize a reference to spi_dev in its drvdata. The result is a
segfault every time the temperature is queried.

Signed-off-by: Graeme Smecher <gsmecher@threespeedlogic.com>
Cc: stable@vger.kernel.org # 3.2+
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>