Low: log: Allow CIB upgrade messages to start with W, I, D

Previously, if cib_upgrade_err() received a message that started with
"W" and ctx == NULL, it would skip the first 9 characters as though the
message began with "WARNING: ". Likewise for "I"/"INFO: " and
"D/DEBUG: ".

The function/logging scheme could be improved further, but this fixes
the immediate issue.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>