rtl818x_pci: add RSSI information for rtl8187SE

This patch makes the driver to report signal strength information
to mac80211 for rtl8187se boards.

It differs from my previous RFT patch:
http://marc.info/?l=linux-wireless&m=140155388332534&w=2


because:
- I have now a working rtl8187se card, so I could serve my RFT by myself. :)
- CCK measurement code has changed a bit, but it does basically the same things.
- OFDM measurement method is changed because the older method reported incorrect
  measures, at least for signals stronger than -40dBm).

CCK measurement seems quite good. OFDM seems less accurate, but this is the
same as the "reference" staging driver dose. I wanted not to change things just
to make measures of _one_ (my) card a bit more close to what _I_ (in my setup)
expected..

IMHO results are still good enough to justify reporting signal in dBm rather than in
"unspecified" units, so this is what this patch actually does.

Results of my tests with a working rtl8187se card connected with coaxes and
various RF attenuators to my AP are:

Input (approx) | CCK meas | OFDM meas
--------------------------------------
      -30dBm   |  -32dBm  |  -31dBm
      -40dBm   |  -40dBm  |  -41dBm
      -50dBm   |  -50dBm  |  -55dBm
      -60dBm   |  -59dBm  |  -63dBm
      -70dBm   |  -69dBm  |  -73dBm
      -80dBm   |  -79dBm  |  -83dBm

Also some real-field tests has been done (no coax, packets in the air) for the CCK
measure method, and they resulted in reasonable values.

Thanks-to: Bernhard Schiffner <bernhard@schiffner-limbach.de> [ for real-field tests]
Signed-off-by: andrea.merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>