wireless-next-2.6: brcm80211: fix compile issue

Commit 59eb21a6 "cfg80211: Extend channel to frequency mapping for 802.11j"
changed the definition of the ieee80211_channel_to_frequency; so fix its
usage in brcmfmac.

Signed-off-by: Stanislav Fomichev <kernel@fomichev.me>
Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>