net: stmmac: Rename clk_ptp_ref clock to ptp_ref

There aren't currently any users of the "clk_ptp_ref", but there are
other references to "ptp_ref", so I'm leaning towards considering that a
typo. Fix it.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>