mwifiex: selectively choose ext_scan support

Some devices do not support extended scan.
This patch adds support to enble ext_scan selectively.
For SD/PCIe interfaces, deefine ext_scan_support as part of
card structure and use it to initialize ext_scan in adapter
during registering device. For USB interfaces, we initialize
ext_scan during register_dev handler.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>