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/axp209.c b/drivers/power/axp209.c
index 731b75e..4b25ef2 100644
--- a/drivers/power/axp209.c
+++ b/drivers/power/axp209.c
@@ -158,7 +158,7 @@
 	ver &= 0x0f;
 
 	if (ver != 0x1)
-		return -1;
+		return -EINVAL;
 
 	/* Mask all interrupts */
 	for (i = AXP209_IRQ_ENABLE1; i <= AXP209_IRQ_ENABLE5; i++) {