stmmac: dwmac-sti: remove clk NULL checks

Since sti_dwmac_parse_data() sets dwmac->clk to NULL if not clock was
provided in DT and NULL is a valid clock there is no need to check for
NULL before using this clock.

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>