ath5k: retain promiscuous setting

ath5k: retain promiscuous setting

Commit 08f75adb96569c28e8b920feae7c9f6a4dc566e7, "ath5k: clean up
filter flags setting" introduced a regression in monitor mode such
that the promisc filter flag would get lost.

Although we set the promisc flag when it changed, we did not
preserve it across subsequent calls to configure_filter.  This patch
restores the original functionality.

Cc: stable@kernel.org
Bisected-by: weedy2887@gmail.com
Tested-by: weedy2887@gmail.com
Tested-by: Rick Farina <sidhayn@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>