hw/ssi/xilinx_spips: Avoid variable length array

In the stripe8() function we use a variable length array; however
we know that the maximum length required is MAX_NUM_BUSSES. Use
a fixed-length array and an assert instead.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190328152635.2794-1-peter.maydell@linaro.org