[NET]: Fix WAN routers kconfig dependency.

Currently WAN router drivers can be built in-kernel while the
register/unregister_wan_device interfaces are built as modules.
This causes:

drivers/built-in.o: In function `cycx_init':
cycx_main.c:(.init.text+0x5c4b): undefined reference to `register_wan_device'
drivers/built-in.o: In function `cycx_exit':
cycx_main.c:(.exit.text+0x560): undefined reference to `unregister_wan_device'
make: *** [.tmp_vmlinux1] Error 1

The problem is caused by tristate -> bool conversion (y or m => y),
so convert WAN_ROUTER_DRIVERS to a tristate so that the correct
dependency is preserved.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wan/Kconfig

@@ -382,7 +382,7 @@ config SDLA
 
 # Wan router core.
 config WAN_ROUTER_DRIVERS
-	bool "WAN router drivers"
+	tristate "WAN router drivers"
 	depends on WAN && WAN_ROUTER
 	---help---
 	  Connect LAN to WAN via Linux box.
@@ -393,7 +393,8 @@ config WAN_ROUTER_DRIVERS
 	  <file:Documentation/networking/wan-router.txt>.
 
 	  Note that the answer to this question won't directly affect the
-	  kernel: saying N will just cause the configurator to skip all
+	  kernel except for how subordinate drivers may be built:
+	  saying N will just cause the configurator to skip all
 	  the questions about WAN router drivers.
 
 	  If unsure, say N.