UBI: get rid of dbg_err

This patch removes the 'dbg_err()' macro and we now use 'ubi_err' instead.
The idea of 'dbg_err()' was to compile out some error message to make the
binary a bit smaller - but I think it was a bad idea.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>