dm: spi: Fix spi_free_slave() freed memory write

Remove setting slave->dev to NULL after the device_remove() call.

The slave pointer points to dev->parent_priv, which has already
been freed by device_free(), called from device_remove() in the
preceding line. Writing to slave->dev may cause corruption of the
dlmalloc free chunk forward pointer of the previously freed chunk.

Signed-off-by: Niel Fourie <lusus@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>