rsi: Fix a potential memory leak in rsi_set_channel()

Fix a potential memory leak in function rsi_set_channel() that is used to
program channel changes. The channel check block for the frequency bands
directly exits the function in case of an error, thus leaving an already
allocated skb unreferenced. Move the checks above allocating the skb.
Detected by Coverity: CID 1195576.

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