mac80211: fix handling of null-rate control in rate_control_get_rate

For hardware with IEEE80211_HW_HAS_RATE_CONTROL the rate controller is not
initialized. However, calling functions such as ieee80211_beacon_get result
in the rate_control_get_rate function getting called, which is accessing
(in this case uninitialized) rate control structures unconditionally.

Fix by exiting the function before setting the rates for HW with
IEEE80211_HW_HAS_RATE_CONTROL set. The initialization of the ieee80211_tx_info
struct is intentionally still executed.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>