samples: pwm: 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: I3c7716fd72e77c99fd29e4e90634c33f47f32e4c
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>