cfg80211: check wiphy driver existence for drvinfo report

When preparing ethtool drvinfo, check if wiphy driver is defined
before dereferencing it. Driver may not exist, e.g. if wiphy is
attached to a virtual platform device.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Link: https://lore.kernel.org/r/20200203105644.28875-1-sergey.matyukevich.os@quantenna.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>