samples: driver: led_apa102c: change hard-coded device name

Use the config name defined by the driver Kconfig in device
binding calls as that is safer because device name can change and
the app does not need to change.

Jira: ZEP-1764

Change-Id: I5a3e16e10f7700ec12edbd07603808cd32f15755
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>