iwlwifi: mvm: rs: fix BT Coex check to look at the correct ant

The check to avoid the shared antenna was passed the wrong
antenna parameter. It should have checked whether the antenna of
the next column we're considering is allowed and instead it was
passed the current antenna.
This could lead to a wrong choice of the next column in the rs
algorithm and non optimal performance.

Fixes: commit 219fb66b ("iwlwifi: mvm: rs - don't use the shared antenna when BT load is high")
CC: <stable@vger.kernel.org> [3.19]
Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>