samples: drivers: i2c fram : 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: 151982

Change-Id: Ifaed47b2b48359dacfdb3111ca2895d5912779e6
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>