P2P: Clear groups first on FLUSH command

P2P: Clear groups first on FLUSH command

This is needed to get proper P2P group removal processing for some test
cases. discovery_group_client followed by nfc_p2p_client was able to hit
a case where the P2P group idle timeout survived to the next group
instance because of the FLUSH command not clearing the group and this
timeout properly.

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