ubi: Fix seq_file usage in detailed_erase_block_info debugfs file

3bfa7e14 ("fs/seq_file.c: seq_read(): add info message about buggy .next functions")
showed that we don't use seq_file correctly.
So make sure that our ->next function always updates the position.

Fixes: 7bccd12d ("ubi: Add debugfs file for tracking PEB state")
Signed-off-by: Richard Weinberger <richard@nod.at>