net: stmmac: Balance PTP reference clock enable/disable

clk_prepare_enable() and clk_disable_unprepare() for this clock aren't
properly balanced, which can trigger a WARN_ON() in the common clock
framework.

Reviewed-By: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>