ubnt,bullet-m and ubnt,rocket-m targets support only single Ethernet
port, therefore disable creation of WAN zone and interfaces for them,
as their intended purpose is a wireless bridge. This aligns the
configuration with one found in ar71xx target.
ubnt,nano-m target is kept as is, being a two-port device.
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
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