[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>