P2Ps: Clear existing value when peer stops sending adv_service_instance

P2Ps: Clear existing value when peer stops sending adv_service_instance

If a peer stops sending adv_service_instance, we should clear the
existing dev->info.p2ps_instance.

This commit fixes the following scenario:

When peer device stops sending adv_service_instance, wpa_supplicant did
not remove old dev->info.p2ps_instance from device's property. This
variable should be updated as per peer behavior and should be cleared
when peer stops sending this information.

Signed-off-by: Nishant Chaprana <n.chaprana@samsung.com>