stmmac: dwmac-sti: move clk_prepare_enable out of init and add error handling

Add clock error handling to probe and in the process move clock enabling
out of sti_dwmac_init() to make this easier.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Tested-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>