drivers: audio: wm8904: Fix missing `default y` in AUDIO_CODEC_WM8904

The `default y`, which is coupled with
`depends on DT_HAS_WOLFSON_WM8904_ENABLED` to link the settings with
DeviceTree, which was missing, so I added it.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>