at86rf230: fix register read for part version

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: Andrey Yurovsky <andrey@snupi.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>