rtl8187: Improve wireless statistics for RTL8187

The current wireless statistics for the RTL8187 poorly indicate the signal
strength and quality. With testing, I found that the AGC value is inversely
correlated with the strength as in the RTL8187B. By implementing a similar
calculation, much more code becomes common to the two devices.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested by: Martín Ernesto Barreyro <barreyromartin@gmail.com>
Acked-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>