brcmutil: add field storing control channel to the struct brcmu_chan

Our d11 code supports encoding/decoding channel info into/from chanspec
format used by firmware. Current implementation is quite misleading
because of the way "chnum" field is used.
When encoding channel info, "chnum" has to be filled by a caller with
*center* channel number. However when decoding chanspec the same field
is filled with a *control* channel number.

1) This can be confusing. It's expected for information to be the same
   after encoding and decoding.
2) It doesn't allow accessing all info when decoding. Some functions may
   need to know both channel numbers, e.g. cfg80211 callback getting
   current channel.
Solve this by adding a separated field for control channel.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>