devlink: Fix error message in devlink_rate_set_ops_supported()

The WARN_ON() macro takes a condition, it doesn't take a message.  Use
WARN() instead.

Fixes: 1897db2e

 ("devlink: Allow setting tx rate for devlink rate leaf objects")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>