ASoC: wcd934x: check sound card state to avoid pointer dereference

When codec probe is not correctly completed and ADSP SSR happens
before sound card is registered, calling SSR callback may result
into NULL pointer dereference and kernel panic happens.
Check sound card state when calling device_down and post_reset to
avoid NULL pointer dereference.

Change-Id: I35fd84af51edf45db6fd451dc68dbcaaed11fee7
Signed-off-by: Meng Wang <mengw@codeaurora.org>