ath9k: fix regression in bssidmask calculation

ath9k: fix regression in bssidmask calculation

The commit that went into 3.17:

    ath9k: Summarize hw state per channel context

    Group and set hw state (opmode, primary_sta, beacon conf) per
    channel context instead of whole list of vifs. This would allow
    each channel context to run in different mode (STA/AP).

    Signed-off-by: Felix Fietkau <nbd@openwrt.org>
    Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
    Signed-off-by: John W. Linville <linville@tuxdriver.com>
broke multi-vif configuration due to not properly calculating
the bssid mask.

The test case that caught this was:

 create wlan0 and sta0-4 (6 total), not sure how much that matters.
 associate all 6 (works fine)
 disconnect 5 of them, leaving sta0 up
 Start trying to bring up the other 5 one at a time.  It will
 fail, with iw events looking like this (in these logs, several
 sta are trying to come up, but symptom is the same with just one)

The patch causing the regression made quite a few changes, but
the part I think caused this particular problem was not
recalculating the bssid mask when adding and removing interfaces.

Re-adding those calls fixes my test case.  Fix bad comment
as well.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>