staging: bcm2835-camera: Add hint about possible faulty config

As per default the GPU memory config of the Raspberry Pi isn't sufficient
for the camera usage. Even worse the bcm2835 camera driver doesn't provide a
helpful error message in this case. So let's add a hint to point the user
to the likely cause.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>