broadcom-wl: fix band reporting

Modify the band reporting for broadcom devices to correctly report
HW capability.  Currently, 802.11na devices report "bgn"; but should be
reporting "an".

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38749 3c298f89-4303-0410-b956-a3cf2f4a3e73