samples: drivers: led_apa102c: Exit from testcase if device not found

Exiting from the test case when the gpio device is not found, this
to avoid a null pointer dereference.

This issue was reported by Coverity

Coverity-CID: 151980

Change-Id: I7aafbc993674c8874a16b5a9213108f5769114ec
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>