zenfone6: Set voice call volume steps to 7

Bug: 28426597
Change-Id: Icf78d9acec75c500982b1e4391af4d6f3933da37
diff --git a/system.prop b/system.prop
index b00dc43..057186c 100644
--- a/system.prop
+++ b/system.prop
@@ -1,5 +1,6 @@
 # Audio
 ro.config.media_vol_steps=25
+ro.config.vc_call_vol_steps=7
 
 # Camera
 persist.camera.capture.hdrplus.dump=0