rt2x00: Fix race between dma mapping and clearing rx entries in rt2800pci

During rx, rt2x00lib calls rt2800pci_fill_rxdone to read the RX
descriptor. At that time the skb is already dma unmapped but no new skb
was dma mapped for this entry again. However, rt2800pci_fill_rxdone also
moves the hw rx queue index, marking this entry to be available for
reuse. Since no new skb was dma mapped and also the previous skb was
unmapped this might lead to strange hw behavior.

To fix this issue move the hw rx queue index increment to
rt2800pci_clear_entry where a new skb was already dma mapped and can be
safely used by the hw.

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