devlink: fix incorrect return statement

A newly added dummy helper function tries to return a failure from a "void"
function:

In file included from include/net/dsa.h:24,
                 from arch/arm/plat-orion/common.c:21:
include/net/devlink.h: In function 'devlink_param_value_changed':
include/net/devlink.h:771:9: error: 'return' with a value, in function returning void [-Werror]
  return -EOPNOTSUPP;

This fixes it by removing the bogus statement.

Fixes: ea601e17 ("devlink: Add devlink notifications support for params")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>