wifi: ath12k: skip sending vdev down for channel switch

Remove the unnecessary WMI vdev down operation as firmware
internally does so already as part of WMI vdev restart.
Repeated vdev down results in a firmware crash during channel
switch operation when multiple BSSID advertisements are enabled:
ath12k_pci 0000:06:00.0: firmware crashed: MHI_CB_SYS_ERROR
ath12k_pci 0000:06:00.0: failed to send WMI_VDEV_SET_PARAM_CMDID
ath12k_pci 0000:06:00.0: failed to set vdev 5 HE MU mode: -108 param_value 7b
ath12k_pci 0000:06:00.0: failed to set he mode vdev 5
ath12k_pci 0000:06:00.0: failed to restart vdev 5: -108
ath12k_pci 0000:06:00.0: failed to send WMI_VDEV_SET_PARAM_CMDID
ath12k_pci 0000:06:00.0: failed to set vdev 4 HE MU mode: -108 param_value 7b
ath12k_pci 0000:06:00.0: failed to set he mode vdev 4
ath12k_pci 0000:06:00.0: failed to restart vdev 4: -108
ath12k_pci 0000:06:00.0: failed to send WMI_VDEV_SET_PARAM_CMDID

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.0.1-00029-QCAHKSWPL_SILICONZ-1

Signed-off-by: Aloka Dixit <quic_alokad@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240508202912.11902-10-quic_alokad@quicinc.com