staging: bcm2835-camera: Replace open-coded idr with a struct idr.

We just need some integer handles that can map back to our message
struct when we're handling a reply, which struct idr is perfect for.

v2: Fix error check to look at the right variable.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>