p54: Add quality output to iwlist and iwconfig

The p54 driver family reports a quality of 0 in iwconfig and iwlist
output. This patch calculates a quality number as a percentage of the
rssi to the maximum signal of 127 reported as the maximum signal.

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