ASoC: wcd9xxx: fix the NULL pointer dereference causing the crash

If the probe of the device is unsuccessful then the wcd9xxx
structure is unallocated but is not initialized to NULL.
If some operation like reset comes, it would dereference the wcd9xxx
structure leading to the target crash.
Fix by intializing the client data to NULL after it is unallocated.

CRs-Fixed: 637697
Change-Id: I2fc58eab0d0a0bfd6907c87c1dacf264c59dce66
Signed-off-by: Aviral Gupta <aviralg@codeaurora.org>