mwifiex: handle extended scan event for AP interface

Stations associated to mwifiex AP would not be able to ping AP
after scan was issued on AP interface. This happened because
there was no handling of extended scan event in AP. This patch
adds this handling and fixes ping failure issue.

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