brcmfmac: determine chanspec from struct cfg80211_chan_def info

The struct cfg80211_chan_def contains additional info to derive the
bandwidth and side-band information of the chanspec. This patch adds
chandef_to_chanspec() function used in IBSS join and starting AP
operation.

Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>