openvswitch: fix building intree datapath module unconditionally

Fixes f2bf8d3 ("openvswitch: provide in-tree kmod builds")

Closes openwrt/packages#7360

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

net/openvswitch/Makefile

@@ -297,7 +297,7 @@ CONFIGURE_VARS += \
 	KARCH=$(LINUX_KARCH) \
 
 ovs_intree_kmod_configs:=CONFIG_PACKAGE_kmod-openvswitch-intree
-ovs_intree_kmod_enabled:=$(strip $(foreach c,$(ovs_intree_kmod_configs),$(c)))
+ovs_intree_kmod_enabled:=$(strip $(foreach c,$(ovs_intree_kmod_configs),$($(c))))
 PKG_CONFIG_DEPENDS+=$(ovs_intree_kmod_configs)
 ifneq ($(ovs_intree_kmod_enabled),)
   CONFIGURE_ARGS += --with-linux=$(LINUX_DIR)