samples: blink_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: If8c14dd4eb186bace863432d454c9122461f2f9c
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>