iwlwifi: move iwl_set_tx_power and make it static

This function is used only in iwl-agn-rxon.c,
move it there and mark it static.

Move this function 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>