iwlwifi: move iwl_connection_init_rx_config to iwl-agn-rxon.c

Move this as part of iwl-core.c cleanup.

Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>