brcmfmac: use nl80211_band directly to get ieee80211 channel

The enum nl80211_band used to retrieve wiphy->bands is the same as
wiphy->bands->band which is checked by wiphy_register(). So it can be used
directly as parameter of ieee80211_channel_to_frequency().

Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>