rtl8180: improve signal reporting for rtl8185 hardware

The existing code seemed to be somewhat based on the datasheet, but
varied substantially from the vendor-provided driver.  This mirrors the
handling of the rtl8185 case from that driver, but still neglects the
specifics for the rtl8180 hardware.  Those details are a bit muddled...

Signed-off-by: John W. Linville <linville@tuxdriver.com>