rt2x00: rt2800usb: add USB device ID for Linksys AE3000

The Linksys AE3000 device is based on the RT3573
chipset. The support for this chipset is available
already, and the AE3000 device works with the driver.

Only managed mode works correctly at the moment,
for AP mode additional changes are needed in the
driver.

Also add a new RT2800USB_RT3573 Kconfig option and
only enable support for RT3573 based devices if
that is enabled.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>