plat/st: correctly check pwr-regulators node

This warning was issued by cppcheck in our downstream code:
[plat/st/common/stm32mp_dt.c:629] -> [plat/st/common/stm32mp_dt.c:634]:
 (warning) Identical condition 'node<0', second condition is always false

The second test has to check variable pwr_regulators_node.

Change-Id: I4a20c4a3ac0ef0639c2df36309d90a61c02b511f
Signed-off-by: Yann Gautier <yann.gautier@st.com>

plat/st/common/stm32mp_dt.c

@@ -469,7 +469,7 @@ uint32_t dt_get_pwr_vdd_voltage(void)
 	}
 
 	pwr_regulators_node = fdt_subnode_offset(fdt, node, "pwr-regulators");
-	if (node < 0) {
+	if (pwr_regulators_node < 0) {
 		INFO("%s: Cannot read pwr-regulators node in DT\n", __func__);
 		return 0;
 	}