iwlwifi: move iwl_set_hw_rfkill_state and mark it static

iwl_set_hw_rfkill_state is used only in
iwl-agn.c. Move it there and mark it static.

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