driver core: platform: Use drv->driver.bus instead of assuming platform_bus_type

In theory (although not *yet* in practice), a driver being passed
to platform_driver_probe might have driver.bus set to something
other than platform_bus_type. Locking drv->driver.bus is always
correct.

Signed-off-by: Patrick Pannuto <ppannuto@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>