net: stmmac: fix warning from Sparse for socfpga

Warning:
drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c:122:41:
sparse: cast removes address space of expression
drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c:122:38:
sparse: incorrect type in assignment (different address spaces)

Signed-off-by: Ley Foon Tan <lftan@altera.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>