nl80211: Extend the new vendor command for testing nl80211

nl80211: Extend the new vendor command for testing nl80211

CONFIG_TESTING_OPTIONS=y build of wpa_supplicant now allows arbitrary
cfg80211 commands to be performed through the new VENDOR ctrl_iface
command by using a special vendor_id ffffffff. The command identifier
(NL80211_CMD_*) is encoded as the subcmd and the attributes in the
hexformatted data area. Response attributes are returned as a hexdump.

For example, this shows a NL80211_CMD_FRAME and a response (cookie
attribute) on a little endian host:

wpa_cli -i wlan0 vendor ffffffff 59 080003004d0000000800260085090000....
0c00580000d7868c0388ffff

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>