at86rf230: remove interframe spacing time workaround

This patch removes the interframe spacing time workaround from at86rf230
driver and use the mac802154 one. The interframe spacing time differs at
at86rf212 and channel setting. This patch fix this handling which is also
a new workaround and should be moved into mac802154 while channel
setting.

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