libertas: fix lbs_rtap attribute in sysfs

At least it doesn't oops when you attempt to read or write it now.
Only when you enable it and then later turn it off. And when it's
enabled I don't see how it actually works.

But one fewer oops is good, for now...

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>