rt2x00: Update copyright statements.

As mentioned on the linux-wireless mailing list, the current copyright
statements in the rt2x00 are meaningless, as the rt2x00 project is
not even a formal legal entity. Therefore it is better to replace
the existing copyright statements with copyright statements for the
people that actually wrote the code.

Note: Updated to the best of my knowledge with respect to who
contributed considerable amounts of code.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
CC: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>