spi: free children in spi_unregister_master, not siblings

introduced by 49dce689 ("spi doesn't need class_device") and bad-fixed
by 350d0076

 ("spi: fix double-free on spi_unregister_master"),
spi_unregister_master would previously device_unregister all of the spi
master's siblings (instead of its children). hilarity ensues.

fix it to unregister children.

Signed-off-by: David Lamparter <equinox@diac24.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>