WNM: Send empty IPv4 packet as keep-alive for now

WNM: Send empty IPv4 packet as keep-alive for now

Use an empty IPv4 packet as the keep-alive packet for WNM BSS max idle
period mechanism. This is not really the best possible frame to use for
this, but for now, it can do until a more suitable frame is figured out
for this (e.g., special LLC header to indicate link test purposes).

Signed-hostap: Jouni Malinen <j@w1.fi>