ath10k: rename WMI_CMD_UNDEFINED

Rename WMI_CMD_UNDEFINED to WMI_CMD_UNSUPPORTED. This is more
accurate here. Also return -EOPNOTSUPP instead of -EINVAL in
wmi_cmd_send().

Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>