Unconfigured bcm43xx device can hit an assert() during wx_get_rate
queries.  This is because bcm43xx calls ieee80211softmac_start late
(i.e. during open instead of probe).

   bcm43xx_net_open ->
      bcm43xx_init_board ->
         bcm43xx_select_wireless_core ->
            ieee80211softmac_start

Fix is to check that device is running before completing
ieee80211softmac_wx_get_rate.
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>