at86rf230: restore trx len when needed

at86rf230: restore trx len when needed

In the most cases the spi messages has a length of two. Currently we
always set the the len field to two before transmit a spi message. In
cases for read out/write in the frame buffer we need another len. This
patch use trx len two as default. For the frame buffer cases we restore
the trx len to two on success and failure. This will reduce the len
setting of two when it's already two.

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