stmmac: fix noderef.cocci warnings

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:115:15-21: ERROR: application of sizeof to pointer

 sizeof when applied to a pointer typed expression gives the size of
 the pointer

Generated by: scripts/coccinelle/misc/noderef.cocci

CC: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>