iwlagn: kill iwlagn_rx_handler_setup

Since iwlagn_rx_handler_setup is always called, fold it into
iwl_rx_handler_setup. BT related handlers are setup by the new
bt_rx_handler_setup lib_ops.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.guy@intel.com>