iwlwifi: Move iwl_send_rxon_timing and make it static

iwl_send_rxon_timing is used only in
iwl-agn-rxon.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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>