at86rf230: fix MAX_CSMA_RETRIES parameter

This patch fix a copy&paste failure for setting the MAX_CSMA_RETRIES
value of the at86rf212 chip which was introduced by commit
f2fdd67c ("ieee802154: enable
smart transmitter features of RF212")

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Cc: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>