brcmsmac: use channel flags to restrict OFDM

brcmsmac cannot call freq_reg_info() during channel changes as it does
not hold cfg80211_lock, and as a result it generates a lockdep warning.
freq_reg_info() is being used to determine whether OFDM is allowed on
the current channel, so we can avoid the errant call by using the new
IEEE80211_CHAN_NO_OFDM for this purpose instead.

Reported-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>