dsa: Allow core and drivers to be built as modules

Change the kconfig types to tristate and adjust the condition for
declaring net_device::dsa_ptr to allow for this.

Adjust the makefile so that if NET_DSA_MV88E6123_61_65=y and
NET_DSA_MV88E6131=m or vice versa then both drivers are built-in.  We
could leave these options as bool and make NET_DSA_MV88E6XXX a
user-selected option, but that would break existing configurations.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 63721a6..87f7353 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1079,7 +1079,7 @@
 #if defined(CONFIG_VLAN_8021Q) || defined(CONFIG_VLAN_8021Q_MODULE)
 	struct vlan_group __rcu	*vlgrp;		/* VLAN group */
 #endif
-#ifdef CONFIG_NET_DSA
+#if IS_ENABLED(CONFIG_NET_DSA)
 	struct dsa_switch_tree	*dsa_ptr;	/* dsa specific data */
 #endif
 	void 			*atalk_ptr;	/* AppleTalk link 	*/