staging: wfx: update copyrights dates

Most of the files have been modified in 2020, so update the copyright
notices.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201007101943.749898-6-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>