UBI: rename ubi_dbg_dump_mkvol_req

I am going to remove the "UBI debugging" compilation option and make the
debugging stuff to be always compiled it. This patch is a preparation
which renames 'ubi_dbg_dump_mkvol_req()' to 'ubi_dump_mkvol_req()'.

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