Add support for the rt2800usb chipset.

Current problems:
 * Cannot scan 11n AP's
 * No TX during first minute after association
 * Broken Hardware encryption

Includes various patches from Mattias, Felix, Xose and Axel.
Signed-off-by: NMattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NXose Vazquez Perez <xose.vazquez@gmail.com>
Signed-off-by: NAxel Kollhofer <rain_maker@root-forum.org>
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>