The controller is registered with serdev when tty ports are registered
with the TTY core. As the TTY core is built-in only, this has the side
effect of making serdev built-in as well.
Signed-off-by: Rob Herring <robh@kernel.org> Reviewed-By: Sebastian Reichel <sre@kernel.org> Tested-By: Sebastian Reichel <sre@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>