cc2520: Make the generated MAC address to look more random

Because the random number generator is very dumb at the moment,
the generated MAC address has always last byte set to 0.
The MAC address is used when generating IPv6 address.
In order to avoid IPv6 address collision, swap the last four
bytes of the MAC address so that the most random data is
at the end of the MAC.

Change-Id: I96c03654359e32f407bab3b29be0e3b08ee91bd7
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>