wifi: rsi: Slightly simplify rsi_set_channel()

There is no point in allocating 'skb' and then freeing it if !channel.

Make the sanity check first to slightly simplify the code.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/29bf0296bd939e3f6952272bfdcc73b22edbc374.1679328588.git.christophe.jaillet@wanadoo.fr