rtl818x_pci: provide dBm signal information for rtl8185

rtl818x_pci: provide dBm signal information for rtl8185

This patch makes the driver report signal information for rtl8185
boards using dBm instead of unspecified unit.
Rtl8180 remains untouched.

I did some tests to confirm the correctness of the measure performed
by the board and it seems reasonably correct.

The test setup has been made by connecting an AP with coax and
RF attenuators to the card antenna port.

In order to get a reference measure I tried with several cards with
different chipset I own. I found that many gave different results, and
I finally selected two cards that gave me consistent results to use
as reference: AR9271 and Prism54-usb (isl3887 with Frisbee radio).

Using this references I compared the RSSI information with my rtl8185
and I repeated tests with three different attenuation values, increasing
attenuation by 10dB each step.
I made only relative measures, making NO assumption about source power.

CCK measures seem very close to my references, OFDM are a little bit
less precise but, considering that these cards are not measuring
instrumentation, IMHO this is still fairly good.

CCK measures (1Mbps beacons)

 ATTENUATOR 1
p54usb:    -58dBm
ath9k_htc: -59dBm
rtl8185:   -59dBm

 ATTENUATOR 2
p54usb:    -67dBm
ath9k_htc: -68dBm
rtl8185:   -70dBm

 ATTENUATOR 3
p54usb:    -78dBm
ath9k_htc: -79dBm
rtl8185:   -79dBm

OFDM measures (54Mbps ping)

 ATTENUATOR 1
p54usb:    -58dBm
ath9k_htc: -57dBm
rtl8185:   -62dBm

 ATTENUATOR 2
p54usb:    -68dBm
rtl8185:   -71dBm

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>