logger: fix --prio-prefix doesn't use --priority default

logger: fix --prio-prefix doesn't use --priority default

The commit b9ef27f have added priority check, but it introduced
regression as the default priority (as specified by --priority) is
ignored.

This patch fixes this problem, but it also removes extra check for
"kern facility", it's unnecessary and inconsistent with the rest of
logger.

Fixes: https://github.com/karelzak/util-linux/issues/1450
Signed-off-by: Karel Zak <kzak@redhat.com>