ath9k: Fix beacon configuration for channel contexts

In channel context mode, when a new context is added,
mac80211 issues a bss_info_changed() notfication when
preparing the connection for the new interface/context.

But, this is done prior to the mgd_prepare_tx() call which
is where we switch to the new context. Since the current
context will be different when the earlier bss_info_changed()
is handled, the beacon information for the VIF is not
updated, but discarded since the rules for the current context
disallows it.

In the subsequent association process for the new context/vif,
this becomes a problem because the beacon parameters are invalid.
This causes problems with the TSF timer, causing large jumps.

To fix this, check if the beacon info is being updated for a
different context and if so, allow it without any checks since
we limit the max. interfaces to two anyway.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>