rsi: Fix a potential memory leak in rsi_send_auto_rate_request()

Fix a potential memory leak in the error path of function
rsi_send_auto_rate_request(). In case memory allocation for array
'selected_rates' fails, the error path exits and leaves the previously
allocated skb in place. Detected by Coverity: CID 1195575.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: John W. Linville <linville@tuxdriver.com>