qcacld-3.0: accept module name externally

qcacld-2.0 to qcacld-3.0 propagation

If module name is provided externally, override the default
module name.

Change-Id: I985f230de601830d8f8c35f1600a09c4ccede88f
CRs-Fixed: 999402

Makefile

@@ -1,7 +1,7 @@
 KERNEL_SRC ?= /lib/modules/$(shell uname -r)/build
 
 KBUILD_OPTIONS := WLAN_ROOT=$(PWD)
-KBUILD_OPTIONS += MODNAME=wlan
+KBUILD_OPTIONS += MODNAME?=wlan
 
 #By default build for CLD
 WLAN_SELECT := CONFIG_QCA_CLD_WLAN=m