at86rf230: remove invalid max frame retries check

This patch removes the invalid max frame retries check from driver
layer. This is already handled by nl802154 framework. Also the IEEE
802.15.4 standard doesn't allow a frame retries setting above 7. This
seems to be valid for the at86rf230 transceiver but the chip running out
of spec then. We only allow settings according 802.15.4 right now.

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