iwlwifi: fix 3945 filter flags

Applying the filter flags directly as done since

commit 3474ad63
Author: Johannes Berg <johannes.berg@intel.com>
Date:   Thu Apr 29 04:43:05 2010 -0700

    iwlwifi: apply filter flags directly

broke 3945 under some unknown circumstances, as
reported by Alex.

Since I want to keep the direct application of
filter flags on iwlagn, duplicate the code into
both 3945 and agn and remove committing the
RXON that broke things from the 3945 version.

Cc: stable@kernel.org [2.6.35]
Reported-by: Alex Romosan <romosan@sycorax.lbl.gov>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>