Handle Wi-Fi Display commands more carefully if P2P is disabled

Handle Wi-Fi Display commands more carefully if P2P is disabled

If P2P was disabled (e.g., due to driver not supporting it or through
p2p_disabled=1 configuration), setting Wi-Fi Display parameters could
result in segmentation fault when the WFD IE is updated without the P2P
module being initialized. Fix this by skipping the update if P2P module
is not in use. In addition, show Wi-Fi Display as disabled in "GET
wifi_display" and refuse to enable it with "SET wifi_display 1" if P2P
is not enabled.

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