power: change from meaningless value to error number

'-1' is absolutely meaningless value.
This patch changed from meaningless value to error number.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/power/pmic/pmic_hi6553.c b/drivers/power/pmic/pmic_hi6553.c
index 0af7987..b2346b6 100644
--- a/drivers/power/pmic/pmic_hi6553.c
+++ b/drivers/power/pmic/pmic_hi6553.c
@@ -26,7 +26,7 @@
 int pmic_reg_write(struct pmic *p, u32 reg, u32 val)
 {
 	if (check_reg(p, reg))
-		return -1;
+		return -EINVAL;
 
 	hi6553_writeb(reg, (uint8_t)val);
 
@@ -36,7 +36,7 @@
 int pmic_reg_read(struct pmic *p, u32 reg, u32 *val)
 {
 	if (check_reg(p, reg))
-		return -1;
+		return -EINVAL;
 
 	*val = (u32)hi6553_readb(reg);