ramips: fix device name of netis WF-2881 to WF2881

The correct model name of WF-2881 is WF2881 without hyphen. The former used
boardnames are not added to SUPPORTED_DEVICES, to make it explicit that the
sysupgrade-tar image, which is newly added in the previous commit, should
not be used to upgrade from older version.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[adjust commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>