net: Make net_hwtstamp_validate accessible

Make the net_hwtstamp_validate function accessible in prevision to use
it from ethtool to validate the hwtstamp configuration before setting it.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>