iwlwifi: mvm: BT Coex - disable BT when TXing probe request in scan

Not doing so will let BT kill our probe requests leading to
failures in scan.

Cc: <stable@vger.kernel.org> [3.10+]
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>