rt2x00: Disable RX aggregation for rt2800usb

RX aggregation is a way to receive multiple 802.11 frames in one RX buffer.
However, we don't know yet how to handle this case in rt2800usb_fill_rxdone
and this has probably no impact on RX performance as well, so we disable it

Signed-off-by: Benoit Papillault <benoit.papillault@free.fr>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>