iwlwifi: use SPDX tags

Use SPDX tags instead of the long copyright notices.  Also cleanup
some duplicate copyright notices and combine the years where possible.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20201210000603.481bcb512a6f.I8146abe5a637079e7336209f23cb26af98b12b31@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>