at86rf230: fix register read for part version

The driver was reading the PART_NUM register for both the part number
(type of device) and the part version, the version is actually in
register 0x1D, VERSION_NUM.  I believe that this was a copy-paste error.

Tested on AT86RF212B where the part is detected to be the expected part
number (0x07) and version (0x03 on mine).
Signed-off-by: NAndrey Yurovsky <andrey@snupi.com>
Acked-by: NAlexander Aring <alex.aring@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

drivers/net/ieee802154/at86rf230.c

@@ -1421,7 +1421,7 @@ at86rf230_detect_device(struct at86rf230_local *lp)
 	if (rc)
 		return rc;
 
-	rc = __at86rf230_read(lp, RG_PART_NUM, &version);
+	rc = __at86rf230_read(lp, RG_VERSION_NUM, &version);
 	if (rc)
 		return rc;