rt2x00: rt2800usb: do not check packedid for aggregated frames

Tx statuses of aggregated subframes contain packetid of first subframe
in the AMPDU. We can not identify AMPDU subframes based on packedid, so
simply assume that status match first pending frame in the queue. Thats
mostly the same what 2800pci do.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>