ARM: 7242/1: PL330: Detach the request from the pl330_thread when it finishes successful
When a request has finished successfully and we are about to call its
callback, remove its pointer from the corresponding pl330_thread .
This prevents the core driver from calling its callback again if
pl330_release_channel() is called without first flushing the device.
When pl330_update() returns, the driver is allowed to free the pointer
to pl330_req so the core driver shouldn't be able to access it again.