driver core: make driver_probe_device() static

It's only used inside drivers/base/dd.c

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Link: https://lore.kernel.org/r/20201123111938.18968-1-jwi@linux.ibm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>