brcmsmac: export firmware version to ethtool

This exports the firmware version in use to userspace through ethtool.

root@OpenWrt:/# ethtool -i wlan0
firmware-version: 610.812

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>