The struct hfi_session_get_property_pkt is always used to fetch a
single property. Make that explicit in the code and avoid a single
element array at the end of the struct.
This change fixes the following cocci warning:
drivers/media/platform/qcom/venus/hfi_cmds.h:194:5-9: WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)