at86rf230: add slp_tr support to start tx

This patch adds support for one of the slp_tr gpio use cases which
indicates the TX_START command without doing some spi bus traffic.

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