samples: sensor: fxos8700: Check sample fetch return value

The sample was reading the sample values regardless of return
value of the fetch sample function, now it exits the function and
prints an error message

Coverity-CID: 157576

Change-Id: If9a909ed9b617357a032c5632ac04984846d6744
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>
1 file changed