ASoC: core: Fix possible NULL pointer dereference

SSR is triggered when the system does not have a sound card
registered, but has codecs. The pointer card sent from codec driver
to snd_soc_card_change_online_state is NULL.
Add null check for soc_card before dereferencing.

Change-Id: I23342a6cbd9d53e62b2ea11dfe79aad3759df6f0
Signed-off-by: Meng Wang <mwang@codeaurora.org>