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.

Reference: <CAJe_ZhftO+481BfL0ErEcM_brfmSuTXkTEniLRYxxM2T7OM2QA@mail.gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>