ath11k: skip sending vdev down for channel switch

The ath11k driver currently sends vdev down to the firmware before
updating the channel context, which is followed by a vdev restart
command.

Sending vdev down is not required before sending a vdev restart,
because the firmware internally does vdev down when ath11k sends
a vdev restart command.

Firmware will happen crash while channel switch without this change.

Hence skip the vdev down command sending when updating the channel
context and then fix the firmware crash issue.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211118095901.8271-1-quic_wgong@quicinc.com