ssb: Add sysfs attributes to ssb devices

Make it possible to read out the attributes, till now only show on
dmesg, through sysfs.

This patch was some time in OpenWrt.

Signed-off-by: Bernhard Loos <bernhardloos@googlemail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>