Clean up setting of iface->p2p_mgmt flag

Clean up setting of iface->p2p_mgmt flag

Previously we set this flag to one in wpa_supplicant_init_iface() if
Wi-Fi controller does not have a dedicated P2P-interface.

This setting had effect only in scope of wpa_supplicant_init_iface() and
it contradicts with comment to struct wpa_interface::p2p_mgmt field.
This comment says that this flag is used only if Wi-Fi controller has
dedicated P2P-device interface.

Also it contradicts with usage of similiar p2p_mgmt field in struct
wpa_supplicant. Again struct wpa_supplicant::p2p_mgmt is set only for
dedicated P2P-device interface.

After this change wpa_interface become input argument to
wpa_supplicant_init_iface() that we are not modifying.

Signed-off-by: Vasyl Vavrychuk <vvavrychuk@gmail.com>