wifi: ath12k: fix overreads in ath12k_wmi_process_csa_switch_count_event()

In the Linux kernel, the following vulnerability has been resolved:

wifi: ath12k: fix overreads in ath12k_wmi_process_csa_switch_count_event()

There is no policy entry for WMI_TAG_PDEV_CSA_SWITCH_COUNT_STATUS_EVENT, so
the parse infrastructure does not enforce a minimum length for the event
struct. Additionally, the num_vdevs field is taken directly from firmware
and used as a loop bound over the vdev_ids array without checking that it
fits within the TLV payload. Either condition can cause an out-of-bounds
read.

Add a TLV policy entry for WMI_TAG_PDEV_CSA_SWITCH_COUNT_STATUS_EVENT so
the parse infrastructure enforces a minimum length for the fixed-size event
struct. Add a helper ath12k_wmi_tlv_data_len() to recover the payload
length of a parsed TLV from the header preceding its data pointer. Use it
in ath12k_wmi_process_csa_switch_count_event() to bound num_vdevs before
the loop.

Compile tested only.