wifi: Add SPDX ids to some files in the wireless subsystem

wifi: Add SPDX ids to some files in the wireless subsystem

Add SPDX-License-Identifier lines to some files where
they are missing in the net/wireless directory.
Remove licensing text from individual files headers (where
present) to canonicalize the library references.

Use (mostly) either GPL-2.0 or ISC, depending on the
license wording (or not) in the files.

radiotap.c does not mention which BSD variant it intends for the
license.  My selection of 'OR BSD-2-Clause' for radiotap.c was
based on research into this code's history and its licensing
elsewhere.  In OpenBSD, radiotap code (which likely either
derived from this code, or this code was derived from) has the
BSD-2-Clause license.  Very similar code in the radiotap library
user space tool, by the same authors Andy Green and Johannes
Berg, has an ISC license. Also the ISC license is used by
Johannes for other contributions in the Linux wireless system.
Since the radiotap.c license text here mentions BSD, but not a
specific version, I chose the closest BSD variant to ISC, which
is BSD-2-Clause.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Link: https://patch.msgid.link/20260305220422.24161-1-tim.bird@sony.com
[modify subject since it's not all radiotap]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>