net: Make dev_get_hwtstamp_phylib accessible

Make the dev_get_hwtstamp_phylib function accessible in prevision to use
it from ethtool to read the hwtstamp current configuration.

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>