iwlwifi: cleanup iwl_set_rxon_channel()

Update outdated comments of iwl_set_rxon_channel() to reflect the
current signature. Also remove the unnecessary validation of the
channel. Those channel info are constructed in iwlwifi driver
and mac80211 will never modify the content of the struct. Also
everytime before this function is called the channel info has
been validated already (as a paranoid check).

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