thermal: int3403_thermal: Downgrade error message

Downgrade "Unsupported event" message from dev_err to dev_dbg to avoid
flooding with this message on some platforms.

Cc: stable@vger.kernel.org # v5.4+
Suggested-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Alex Hung <alex.hung@canonical.com>
[ rzhang: fix typo in changelog ]
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200615223957.183153-1-alex.hung@canonical.com