iwlwifi: move iwlagn_hw_valid_rtc_data_addr prototype

Since this function is now used only by
op_mode, remove it from iwl-shared.h and move
it to iwl-agn.h.

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