ath5k: retain promiscuous setting

ath5k: retain promiscuous setting

Commit 56d1de0a21db28e41741cfa0a66e18bc8d920554, "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>