at86rf230: rework detect device handling

This patch drops the current lowlevel spi calls for the detect device
function instead we handle this via regmap. Also put the detection of
in a seperate function and set all device specific attributes while detection.

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