ubi: fastmap: Fix inverted logic in seen selfcheck

set_seen() sets the bit corresponding to the PEB number in the bitmap,
so when self_check_seen() wants to find PEBs that haven't been seen we
have to print the PEBs that have their bit cleared, not the ones which
have it set.

Fixes: 5d71afb0 ("ubi: Use bitmaps in Fastmap self-check code")
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>