wifi: mt76: relicense to BSD-3-Clause-Clear

wifi: mt76: relicense to BSD-3-Clause-Clear

MediaTek has asked to switch from the ISC license to BSD-3-Clause-Clear,
in order to improve clarity and the legal integrity of the code.

The BSD-3-Clause license includes the "no endorsement" clause, which is
important for protecting the reputation of the original authors and
contributors by preventing unauthorized use of their names for endorsement
purposes.

This clause is absent in the BSD-2-Clause license, which is more permissive
but lacks this specific protection.

This change also cleans up the license of some Kconfig/Makefile files,
which were accidentally marked as GPL.

The GPL 2.0 remains in use on mt76x0, as well as two source files in mt7615
for which the license situation still needs to be clarified.

Link: https://patch.msgid.link/20251008104250.46292-2-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>