ath5k: retain promiscuous setting

Commit 56d1de0a, "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>