mediatek: remove condition in Device/Default

mediatek: remove condition in Device/Default

The current condition with part of the variables set dependent on
the subtarget in Device/Default isn't really nice to read and also
defeats the purpose of having a default node.

This removes the special settings for mt7623 and moves them to the
individual devices, which is not much of a problem as there are
actually just two of them and they partly use different settings
anyway.

While at it, slightly adjust the order of variables and wrap some
long lines.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>