mwifiex: do not initialize ext_scan in mwifiex_init_adapter

Features which are device specific are already updated in
interface specific initialization e.g. register_dev.
We should not initialize them in mwifiex_init_adapter();
else this would overwrite earlier settings.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>