driver core: platform: reorder functions

This way all callbacks and structures used to initialize
platform_bus_type are defined just before platform_bus_type and in the
same order. Also move platform_drv_probe_fail just before it's only
user.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20201119124611.2573057-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>