mwifiex: move if_ops.cleanup_if() call

As if_ops.init_if() is called in mwifiex_register(),
corresponding cleanup routine should be called in
mwifiex_unregister().

Currently it's there in mwifiex_adapter_cleanup(), hence
interface specific cleanup is not performed if driver
initialization is failed.

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>