We get RXORN interrupts when all receive buffers are full. This is not
necessarily a fatal situation. It can also happen when the bus is busy or the
CPU is not fast enough to process all frames.

Older chipsets apparently need a reset to come out of this situration, but on
newer chips we can treat RXORN like RX, as going thru a full reset does more
harm than good, there.

The exact chip revisions which need a reset are unknown - this guess
AR5K_SREV_AR5212 ("venice") is copied from the HAL.

Inspired by openwrt 413-rxorn.patch:
"treat rxorn like rx, reset after rxorn seems to do more harm than good"
Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>