carl9170: export HW random number generator

All AR9170 hardware have a 16-Bit random number generator.
The documentation claims the values are suitable for
"security keys".

The "throughput" is around 320Kibit/s. It's slow, but it
does work without introducing any special offload
firmware commands.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>