samples: drivers: gpio: 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: I44f13131d44c7c093781e1f11f8481e7ef8175c9
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>