ath9k: Group Key fix for VAPs

When I set up multiple VAPs with ath9k, I encountered an issue that
the traffic may be lost after a while.

The detailed phenomenon is
1. After a while the clients connected to one of these VAPs will get
into a state that no broadcast/multicast packets can be transfered
successfully while the unicast packets can be transfered normally.
2. Minutes latter the unitcast packets transfer will fail as well,
because the ARP entry is expired and it can't be freshed due to the
broadcast trouble.

It's caused by the group key overwritten and someone discussed this
issue in ath9k-devel maillist before, but haven't work out a fix yet.

I referred the method in madwifi, and made a patch for ath9k.
The method is to set the high bit of the sender(AP)'s address, and
associated that mac and the group key. It requires the hardware
supports multicast frame key search. It seems true for AR9160.

Not sure whether it's the correct way to fix this issue. But it seems
to work in my test. The patch is attached, feel free to revise it.

Signed-off-by: Daniel Yingqiang ma <yma.cool@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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