rtl8187: Improve wireless statistics for RTL8187B

Wireless statistics produced by the RTL8187B driver are not particularly
informative about the strength of the received signal. From the data sheet
provided by Realtek, I discovered that certain parts of the RX header
should have the information necessary to calculate signal quality and
strength. With testing, it became clear that most of these quantities were
very jittery - only the AGC correlated with the signals expected from nearby
AP's. As a result, the quality and strength are derived from the agc value.
The scaling has been determined so that the numbers are close to those
obtained by b43 under the same conditions. The results are qualitatively
correct.

Statistics derived for the RTL8187 have not been changed.

The RX header variables have been renamed to match the quantites described
in the Realtek data sheet.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>