qcacld-3.0: Enable CONFIG_CNSS_WCN7850 in WCN7850 config file

Device specific flags should be enabled from Device specific config
file itself. They don't need to be passed from upper level Makefiles.

Change-Id: Ic05cebb225cf63a6163f64cc72c02f38d2d25231
CRs-fixed: 2944594

configs/wcn7850_defconfig

@@ -1 +1,3 @@
+CONFIG_CNSS_WCN7850 := y
+
 include $(WLAN_ROOT)/configs/default_defconfig