include: clock_control: Fix int return value

The return value of clock_control_get_status is a clock_control_status
enum, but in case the device is not ready, an int is returned.
This leads to compiler errors. Introduced a new enum value
CLOCK_CONTROL_STATUS_UNAVAILABLE and return it instead of -ENODEV.

Signed-off-by: Alexander Wachter <alexander.wachter@leica-geosystems.com>