at86rf230: fix cca ed values for rf233

This patch changes the rssi base value to -94 for at86rf33 transceivers.
The code before assumes a rssi base value of -91 which is for the
at86rf231 transceiver only. This change need to update the cca ed
threshold mapping table.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>