certs: don't try to update blacklist keys

When the same key is blacklisted repeatedly logging at pr_err() level is
excessive as no functionality is impaired.
When these duplicates are provided by buggy firmware there is nothing
the user can do to fix the situation.
Instead of spamming the bootlog with errors we use a warning that can
still be seen by OEMs when testing their firmware.

Link: https://lore.kernel.org/all/c8c65713-5cda-43ad-8018-20f2e32e4432@t-8ch.de/
Link: https://lore.kernel.org/all/20221104014704.3469-1-linux@weissschuh.net/


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>