UBI: add in hex the value for UBI_INTERNAL_VOL_START to comment

Explicitly provide the first internal volume ID value in the comment for
UBI_INTERNAL_VOL_START. This allows developers who, when adding features
related to volume ids and observe unexpected very large volume ids, to grep
for the observed value in the source code and find out immediately that it is
expected behaviour.

Signed-off-by: Joel Reardon <reardonj@inf.ethz.ch>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>