cfg80211: Allow NL80211_ATTR_IFINDEX to be added to vendor events

This modifies cfg80211_vendor_event_alloc() with an additional argument
struct wireless_dev *wdev. __cfg80211_alloc_event_skb() is modified to
take in *wdev argument, if wdev != NULL, both the NL80211_ATTR_IFINDEX
and wdev identifier are added to the vendor event.

These changes make it easier for drivers to add ifindex indication in
vendor events cleanly.

This also updates all existing users of cfg80211_vendor_event_alloc()
and __cfg80211_alloc_event_skb() in the kernel tree.

Signed-off-by: Ahmad Kholaif <akholaif@qca.qualcomm.com>
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>