mlxsw: core: Change BUG to WARN in hwmon code

Better to just warn the user that something really odd is going on and
continue to run.

Suggested-by: Or Gerlitz <gerlitz.or@gmail.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>