at86rf230: add support for at86rf23x desense

To set the CCA_ED_THRES register the calculation for at86rf23x is
different than for at86rf212. This patch adds a new callback for this
calculation in chip data struct.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>