Blackfin: bf537: rename "CONFIG_ADT75"

In v3.2 the Analog Devices ADT75 temperature sensor driver was removed
as an IIO driver and support for it was added to the LM75 HWMON driver.
But it was apparently overlooked to rename one reference to CONFIG_ADT75
to CONFIG_SENSORS_LM75. Do so now. Use the IS_ENABLED() macro, while
we're at it.

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