mwifiex: inform cfg80211 about disconnect for P2P client interface

This patch adds missing cfg80211_disconnected event for P2P client
interface upon successful deauthenticate command, deauthenticate
event or disassociate event from FW.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>