iwlwifi: fix set_tx_power vs scan

According to comment in iwl_bg_scan_completed, setting tx power should
be deferred during pending scan, but we are not doing this.

This patch change code to really defer setting tx power after scan
complete. Additionally refactor iwl_set_tx_power code and call
lib->send_tx_power() directly from iwlagn_commit_rxon.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by:  Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>