rt2x00: Implement TX status reporting for rt2800usb

The TX_STA_FIFO register which is used for per-frame TX frame
status reporting is also valid on rt2800usb. We can move the
rt2800pci_txdone function into rt2800lib where it can also
be used by rt2800usb.

rt2800usb needs to overwrite the txdone work handler to
a different function.

Both rt2800usb as rt2800_txdone need to take into account
that IO failures can occur while uploading the URB, which
means that when obtaining the new entry the IO status must
be checked.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>