rt2x00: Limit rt2x00pci rxdone processing to 16 entries at once

Instead of receiving an unlimited number of frames, stop after 16
entries and reschedule the rxdone tasklet. This allows other tasklets
to be run inbetween.

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