wcd934x: Fix NULL pointer dereference for wcd9xxx pointer

During bind/unbind of ngd_msm_ctrl power_gate_digital_core
is called. Simultaneously tavil_remove is also called
making tavil pointer as NULL.  Check for tavil as not NULL
at the start of function to avoid NULL pointer dereference.

CRs-Fixed: 2267796
Change-Id: I6fc476cc8b706b556836f30838983de0f34d4fc1
Signed-off-by: Vatsal Bucha <vbucha@codeaurora.org>

asoc/codecs/wcd934x/wcd934x.c

@@ -8939,6 +8939,8 @@ static struct snd_soc_dai_driver tavil_i2s_dai[] = {
 
 static void tavil_codec_power_gate_digital_core(struct tavil_priv *tavil)
 {
+	if (!tavil)
+		return;
 	mutex_lock(&tavil->power_lock);
 	dev_dbg(tavil->dev, "%s: Entering power gating function, %d\n",
 		__func__, tavil->power_active_ref);