mac80211: Add more ethtools stats: survey, rates, etc

The signal and noise are forced to be positive since ethtool
deals in unsigned 64-bit values and this number should be human
readable.  This gives easy access to some of the data formerly
exposed in the deprecated /proc/net/wireless file.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>