at86rf230: correct aret lifs and sifs handling

This patch adds lifs/sifs handling only if max_frame_retries is above
zero. The at86rf2xx datasheets says nothing about phy lifs/sifs
handling. I asked the atmel support and they said lifs/sifs is done
by phy when max_frame_retries is above zero.

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