spi: Fix device unregistration when unregistering the bus master

spi: Fix device unregistration when unregistering the bus master

commit 178db7d30f94707efca1a189753c105ef69942ed upstream.

Device are added as children of the bus master's parent device, but
spi_unregister_master() looks for devices to unregister in the bus
master's children. This results in the child devices not being
unregistered.

Fix this by registering devices as direct children of the bus master.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>