stmmac: Remove unbalanced clk_disable call

The stmmac_open call was calling clk_disable_unprepare on phy init
failure, but it never calls clk_prepare_enable, this causes
a WARN_ON in the clk framework to trigger if for some reason phy init
fails.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>