driver core: platform: fix race condition with driver_override

The driver_override implementation is susceptible to race condition when
different threads are reading vs storing a different driver override.
Add locking to avoid race condition.

Fixes: 3d713e0e ("driver core: platform: add device binding path 'driver_override'")
Cc: stable@vger.kernel.org
Signed-off-by: Adrian Salido <salidoa@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>