rt2x00: Move duplicate code into rt2x00pci_txdone()

rt2400pci, rt2500pci and rt61 require different
txdone handling, but the code that pushes the frame
upstream and cleans up the entry is identical to
all of them.
This will create the function rt2x00pci_txdone()
to remove the duplicate code.

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