driver core: platform: change logic implementing platform_driver_probe

Instead of overwriting the core driver's probe function handle probing
devices for drivers loaded by platform_driver_probe() in the platform
driver probe function.

The intended goal is to not have to change the probe function to
simplify converting the platform bus to use bus functions.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20201119124611.2573057-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>