Revert "b43: trivial: do not report any link quality instead of invalid one"

This reverts commit 55ad5962.

I assumed N is newer than LP, which isn't true. This regressed LP case.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>