ASoC: rt286: Fix potencial crash in jd function

We assign rt286->codec in rt286_probe. If rt286_jack_detect is
invoked before rt286_probe, rt286->codec will be NULL and cause
a kernel panic.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>