ptp: ptp_clock: Publish scaled_ppm_to_ppb

[ Upstream commit 4368dada

 ]

Publish scaled_ppm_to_ppb to allow drivers to use it.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>