[third_party/bcmdhd] Chanspec conversion logic. Extract logic for firmware chanspec conversion, including logic for channel configurations with bandwidth greater than 20 MHz. Change-Id: I6ba54e90b603853c2ba7a0b65e4a931aa5638ab0 Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/bcmdhd/+/1322645 Reviewed-by: John Tan <johntan@google.com>