rt2x00: rt2800usb: rework txdone code

Patch change txdone code to make it similar like txdone in rt2800pci,
process only one entry from queue matching tx status.

Before we processed all pending entries from queue until PACKEDID match,
that caused that we do not report tx statuses correctly.

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