ASoC: wcd: don't set autozeroing for conga

Conga analog codec has fix to support special headset detection
and button press. So no need to set autozeroing for conga.
Check for function pointer before accessing it.

Change-Id: I52a79a50d8f0a45d7320e63ed753050178ad2951
Signed-off-by: Yeleswarapu Nagaradhesh <nagaradh@codeaurora.org>