cfg80211: add helper fn for adjacent rule channels

Some usable channels are located in the union of adjacent
regulatory rules, for example channel 144 in Germany.

Enable them, by also checking if a channel spans two adjacent
regulatory rules/frequency ranges.

All flags involved are disabling things, therefore we can build
the maximum by or-ing them together. Furthermore, take the maximum
of DFS CAC time values and the minimum of allowed power of both
adjacent channels in order to comply with both regulatory rules at
the same time.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20200803144353.305538-2-markus.theil@tu-ilmenau.de
[remove unrelated comment changes]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>