brcmfmac: deal with NL80211_CHANWIDTH_20_NOHT setting

The driver can get a channel definition with bandwidth set to
NL80211_CHANWIDTH_20_NOHT. This was no handled in brcmfmac which
results in a warning splat. One scenario in which this is observed
was when running hostapd with configuration 'hw_mode=g'.

Cc: Jon Nettleton <jon.nettleton@gmail.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>