at76c50x-usb: update to latest mac80211 hw scan api

With the latest mac80211 stack, the driver needs to be updated for
cfg80211 scanning.  I based the changes off of modifications for
at76_usb found here:

http://johannes.sipsolutions.net/patches/old/all/2008-09-19-13:35/020-cfg80211-scan.patch



The trick was that max_signal also needs to be set to avoid a divide
by zero Oops.  I just guessed and used the value 100 for now.

kvalo: handpicked the change from two different patches

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>