rt2x00: Cleanup RX index counting

Add the rt2x00_dmastart function to rt2x00lib which
marks the queue_entry as "owned by device", and increased
the Q_INDEX number.

This cleanups up the index handling by rt2x00lib which
at until so far used hackish approaches to keep the
RX queue index numbering sane.

The rt2x00pci.c changes are from Helmut Schaa

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