cfg80211: separate get channel number from ies

Get channel number from ies is a common logic, so separate it to a new
function, which could also be used by lower driver.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Link: https://lore.kernel.org/r/20210930081533.4898-1-wgong@codeaurora.org


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