rt2x00: Implement support for rt2800usb

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: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Signed-off-by: Axel Kollhofer <rain_maker@root-forum.org>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>