iwlwifi: use ieee80211_frequency_to_channel

This patch replaces ieee80211chan2mhz from radiotap with
ieee80211_frequency_to_channel provided by mac80211

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>