[MAC80211]: Add support for SIOCGIWRATE ioctl

At present, transmission rate information for mac80211 is available only
if verbose debugging is turned on, and then only in the logs. This patch
implements the SIOCGIWRATE ioctl, which adds the current transmission rate to
the output of iwconfig.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>