iwlwifi: do not call ieee80211_frequency_to_channel

A few cases in iwlwifi driver function ieee80211_frequency_to_channel()
is called to get channel number from center frequency. This is not needed
since the channel number is already saved in hw_value field of struct
ieee80211_channel in function iwlcore_init_geos(). So replace those function
calls with hw_value field of struct ieee80211_channel.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>