samples: fade_led: change hard-coded device name

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

Jira: ZEP-1764

Change-Id: I3287da5c5a9df24507efa84bbf7bbb051726bc2c
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>