iwlwifi: introduce iwl_advanced_bt_coexist()

We use priv->cfg->bt_params && priv->cfg->bt_params->advanced_bt_coexist
conditional in few places, merge it into one function.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>