DFS: Consider non-contiguous channels

DFS: Consider non-contiguous channels

When looking for a new operating channel, consider the case of
non-contiguous channels when checking all the needed channels (e.g., the
driver might support channels 36, 38, 40, so look for channels 36+40
explicitly, instead of failing when encountering channel 38).

Signed-off-by: Eliad Peller <eliad@wizery.com>