ACPI / scan: Simplify ACPI driver probing

There is no particular reason why acpi_bus_driver_init() needs to be
a separate function and its location with respect to its only caller,
acpi_device_probe(), makes the code a bit difficult to follow.

Besides, it doesn't really make sense to check if 'device' is not
NULL in acpi_bus_driver_init(), because we've already dereferenced
dev->driver in acpi_device_probe() at that point and, moreover,
'device' cannot be NULL then, because acpi_device_probe() is called
via really_probe() (which also sets dev->driver for that matter).

For these reasons, drop acpi_bus_driver_init() altogether and move
the remaining code from it directly into acpi_device_probe().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>