sensor: fixes dead code in the apds9960 sample

If the gpio or spi devices are not found there is no
need to keep the device busy in a loop for this particular
sample.

Since it is not possible to continue execution it is better
to simply end the application.

Change-Id: Ie25ea970a479db2a2f339ca2b37f88541a45ef97
Coverity-CID: 151973
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@intel.com>