at76c50x-usb: set firmware and hardware version in wiphy

Set firmware and hardware version in wiphy so that user space can access
it.

(Modification from original in favor of cfg80211 ethtool support. -- JWL)

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