ASoC: wcd9306: fix the NULL pointer dereference in the tapan codec

During the close of the AFE port the AIF1 PB widget will be closed
which access the core variable.
If the core is not initialized yet it will result in the kernel panic.
Fix by adding the check for the core structure to be initialized.

CRs-Fixed: 635790
Change-Id: Ic057e2eb75b25dbc55aa06cbce2b49aca0b1b25f
Signed-off-by: Aviral Gupta <aviralg@codeaurora.org>