Add QCA vendor attribute and event to indicate subnet change status

Add QCA vendor attribute and event to indicate subnet change status

This allows offloaded roaming to inform user space of the change in IP
subnet post roaming. The device may have roamed to a network which is in
a different subnet which will result in IP connectivity loss. Indicating
the change in subnet enables the user space to refresh the IP address or
to perform IP subnet validation if unknown status is indicated.

The driver indication is reported with a new event from wpa_supplicant
in the following format:
CTRL-EVENT-SUBNET-STATUS-UPDATE status=<0/1/2>
where
0 = unknown
1 = IP subnet unchanged (can continue to use the old IP address)
2 = IP subnet changed (need to get a new IP address)

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