brcm80211: smac: use current regulatory domain when checking whether OFDM is allowed

The brcmsmac internal regulatory data is being used to determine whether
OFDM should be allowed, and this is only done once during
initialization. To be effective this needs to be checked against
mac80211's regulatory rules for the current channel.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>