hw/ssi/pnv_spi: Match _xfer_buffer_free() with _xfer_buffer_new()

hw/ssi/pnv_spi: Match _xfer_buffer_free() with _xfer_buffer_new()

pnv_spi_xfer_buffer_new() allocates %payload using g_malloc0(),
and pnv_spi_xfer_buffer_write_ptr() allocates %payload->data
using g_realloc(). Use the API equivalent g_free() to release
the buffers.

Cc: qemu-stable@nongnu.org
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>