power: regulator: stm32: Fix warnings when compiling with W=1

This patch solves the following warnings:

drivers/power/regulator/stm32-vrefbuf.c: In function 'stm32_vrefbuf_set_value':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (uV == stm32_vrefbuf_voltages[i]) {
          ^~

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/power/regulator/stm32-vrefbuf.c b/drivers/power/regulator/stm32-vrefbuf.c
index 0ad6833..645528e 100644
--- a/drivers/power/regulator/stm32-vrefbuf.c
+++ b/drivers/power/regulator/stm32-vrefbuf.c
@@ -30,7 +30,7 @@
 	struct udevice *vdda_supply;
 };
 
-static const unsigned int stm32_vrefbuf_voltages[] = {
+static const int stm32_vrefbuf_voltages[] = {
 	/* Matches resp. VRS = 000b, 001b, 010b, 011b */
 	2500000, 2048000, 1800000, 1500000,
 };