netdevice pc300: Add the reason about PC300 BROKEN in Kconfig

When compile test my previous patch, I found PC300 driver was broken.
And there is no explanation about the broken.
Add the reason about why change this driver to broken in Kconfig.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>