UBI: make it possible to use struct ubi_device in debug.h

Current layout does not allow us to add inline functions to debug.h which use
the 'struct ubi_device' object, because it is undefined there. Move
'#include "debug.h"' in "ubi.h" down so to make 'struct ubi_device" be defined.
Additionally, this makes it possible to remove a bunch of forward declarations
in "debug.h". This is a preparation to the next patch.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>