power: stpmic1: Fix warnings when compiling with W=1

This patch solves the following warnings:
warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/power/regulator/stpmic1.c b/drivers/power/regulator/stpmic1.c
index 50ef2a2..1e3f96f 100644
--- a/drivers/power/regulator/stpmic1.c
+++ b/drivers/power/regulator/stpmic1.c
@@ -422,6 +422,7 @@
 	case STPMIC1_LDO_MODE_SINK_SOURCE:
 		ret &= ~STPMIC1_LDO12356_VOUT_MASK;
 		ret |= STPMIC1_LDO3_DDR_SEL << STPMIC1_LDO12356_VOUT_SHIFT;
+		/* fallthrough */
 	case STPMIC1_LDO_MODE_NORMAL:
 		ret &= ~STPMIC1_LDO3_MODE;
 		break;