drivers/xpowers/axp209: Adapt to new I²C API

Originally, axp209_(read|write) accessors relied on i2c_(read|write)
to return the number of bytes transferred. This was changed in
* cdb61a6 i2c: Replace the i2c API.
to return an error code or 0 on success. This caused the AXP209 check
to fail. Fix the accessors to account for this new behavior.

Change-Id: Ib0f492bd52260d224d87f8e8f2d3c1244d1507df
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/8432
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins)
diff --git a/src/drivers/xpowers/axp209/axp209.c b/src/drivers/xpowers/axp209/axp209.c
index ae2e3da..f36c506 100644
--- a/src/drivers/xpowers/axp209/axp209.c
+++ b/src/drivers/xpowers/axp209/axp209.c
@@ -43,16 +43,22 @@
  * is no limitation on the AXP209 as to how many registers we may read or write
  * in one transaction.
  * These return the number of bytes read/written, or an error code. In this
- * case, they return 1 on success, or an error code otherwise.
+ * case, they return 1 on success, or an error code otherwise. This is done to
+ * work with I²C drivers that return either 0 on success or the number of bytes
+ * actually transferred.
  */
 static int axp209_read(u8 bus, u8 reg, u8 *val)
 {
-	return i2c_readb(bus, AXP209_I2C_ADDR, reg, val);
+	if (i2c_readb(bus, AXP209_I2C_ADDR, reg, val) < 0)
+		return CB_ERR;
+	return 1;
 }
 
 static int axp209_write(u8 bus, u8 reg, u8 val)
 {
-	return i2c_writeb(bus, AXP209_I2C_ADDR, reg, val);
+	if (i2c_writeb(bus, AXP209_I2C_ADDR, reg, val) < 0)
+		return CB_ERR;
+	return 1;
 }
 
 /**