OpenWrt in default uses the reverse engineered b43 driver for BCM47xx and BCM53xx devices. With this driver only 802.11g is supported, but you can easily get wireless 802.11n working by installing the Broadcom open source brcmsmac driver.
You can easily do make wireless N working by following below steps.
Step 1, telnet or SSH to the router.
Step 2. execute the following commands through SSH or telnet.
By running the following commands you will remove kmod-b43 and other unnecessary drivers, install kmod-brcmsmac and kmod-brcmutil package.
opkg update opkg install kmod-brcmsmac opkg install kmod-brcmutil rmmod b43 rmmod b43legacy rmmod wl rmmod brcmsmac rmmod brcmutil modprobe brcmsmac
Step 3, delete the old config files and get the initial configuration file.
rm -f /etc/config/wireless wifi detect > /etc/config/wireless
Step 4, enable the WiFi.
Comment out "option disabled 1" by adding "#" in front of it or delete it "option disabled 1" completely.
After that, run the following command and you will see SSID named OpenWrt on your other devices.
*Note: brcmsmac does not support 40mhz.