ToDevice.u: Only select default METHOD if supported.

Fix bug introduced in 3d46d1b5, triggered if the methods enabled by
TODEVICE_ALLOW_* and FROMDEVICE_ALLOW_* do not match, in which case a
default METHOD could be selected that was only enabled for FromDevice.u
and not for ToDevice.u, causing ToDevice.u to silently drop packets.
ToDevice.u: Document METHOD values and defaults.