wcn36xx: Uninitialized variable in wcn36xx_change_opchannel()

This code needs "channel" to be initialized to NULL for it to work
correctly.

Fixes: d6f27466 ("wcn36xx: Track the band and channel we are tuned to")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonghue@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220201125941.GA22458@kili