ASoC: rt5651: Add rt5651_jack_inserted() helper

Add rt5651_jack_inserted() helper to get the jack-detect switch status,
This is a preparation patch for rewriting the jack type-detection to
make it more reliable.

Tested-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mark Brown <broonie@kernel.org>