wcn36xx: Remove useless skb spinlock

Each DXE control block is associated to a specific channel.
The channel lock is always taken before accessing a control block.
There is no need to have an extra (useless) spinlock for the control
block skb.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>