staging/bcm2835-isp: Fix cleanup after init fail

bcm2835_isp_remove is called on an initialisation failure, but at that
point the drvdata hasn't been set. This causes a crash when e.g. using
the cutdown firmware (gpu_mem=16).

Move platform_set_drvdata before the instance probing loop to avoid the
problem.

See: https://github.com/raspberrypi/linux/issues/4774



Signed-off-by: Phil Elwell <phil@raspberrypi.com>