net: add missing SPDX-License-Identifier for files originating from LiMon

net: add missing SPDX-License-Identifier for files originating from LiMon

The header of LiMon imported files reference a License file which
does not exist in U-Boot. Some files were forgotten when adding the
SPDX-License-Identifier.
The LiMon files were originally licensed under GPLv2 as can be seen in
commit [2ea91039].

Based on this commit, add the correct SPDX license identifier.
While at it drop the reference to the non-existing License file from all
LiMon files and update the SPDX-License-Identifier to SPDX version 3.

Signed-off-by: Max Merchel <Max.Merchel@ew.tq-group.com>