ath10k: fix WMI scan command length

Fix WMI scan command length we setup when scan request.

This fix issue with 636 firmware when scan always failed
with message:

ath10k_pci 0000:02:00.0: wmi start scan
ath10k_pci 0000:02:00.0: wmi stop scan reqid 1 req_type 0 vdev/scan_id 0
ath10k_pci 0000:02:00.0: failed to stop wmi scan: -11
ath10k_pci 0000:02:00.0: failed to stop scan: -11
ath10k_pci 0000:02:00.0: failed to start hw scan: -110

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>