cfg80211: refactor cfg80211_get_ies_channel_number()

Now that this is no longer part of the bigger function,
we can get rid of the channel_num variable. Also change
the function to use the struct element helpers, instead
of open-coding the element handling.

Link: https://lore.kernel.org/r/20211202130913.a0adf67a9319.I6db0340a34fff18d78e9cd512f4abf855da4e43a@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>