cfg80211: add helper fn for single rule channels

As a preparation to handle adjacent rule channels,
factor out handling channels located in a single
regulatory rule.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20200803144353.305538-1-markus.theil@tu-ilmenau.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>