UBI: gluebi: rename misleading variables

Both names 'total_read' and 'total_written' are actually used
as the number of bytes left to read and write.
Fix this confusion by renaming both to 'bytes_left'.

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