iwlwifi: mvm: fix antenna selection when BT is active

When BT is active, we want to avoid the shared antenna for
management frame to make sure we don't disturb BT. There
was a bug in that code because it chose the antenna
BIT(ANT_A) where ANT_A is already a bitmap (0x1). This
means that the antenna chosen in the end was ANT_B.
While this is not optimal on devices with 2 antennas (it'd
disturb BT), it is critical on single antenna devices like
3160 which couldn't connect at all when BT was active.

This fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=97181



CC: <stable@vger.kernel.org> [3.17+]
Fixes: 34c8b24f ("iwlwifi: mvm: BT Coex - avoid the shared antenna for management frames")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>