drivers: sensor: Fix apds9960 device pointer initialization

Fixes a bus fault observed in samples/boards/reel_board/mesh_badge
caused by dereferencing an uninitialized device pointer in the apds9960
sensor driver. The device pointer needs to be initialized regardless of
how CONFIG_APDS9960_TRIGGER is configured.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>