wlan: update project discrimination in Android.mk

The prima and qcacld-2.0 projects both use the WLAN_CHIPSET variable
to keep track of whether or not the given project is supported on a
given platform.  The problem is that both of these assume WLAN_CHIPSET
is initially empty, but that is not always the case.  When both are
included in an Android build, the value may be set by one of the
projects, and that value will be inherited by the other project.  To
prevent this explicitly clear the WLAN_CHIPSET variable in both
projects.

Change-Id: I138efff35a1d82e5e13e43b603df350b092d4fb5
diff --git a/Android.mk b/Android.mk
index 795064b..3ccf472 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1,5 +1,8 @@
 # Android makefile for the WLAN Module
 
+# Assume no targets will be supported
+WLAN_CHIPSET :=
+
 # Build/Package options for 8960 target
 ifeq ($(call is-board-platform,msm8960),true)
 WLAN_CHIPSET := prima