Blackfin: bf537: rename "CONFIG_AD7314"

In v3.2 the Analog Devices AD7314 temperature sensor driver was removed
as an IIO driver and added as a HWMON driver. But it was apparently
overlooked to rename two references to CONFIG_AD7314 to
CONFIG_SENSORS_AD7314. Do so now. Use the IS_ENABLED() macro, while
we're at it.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>