mwifiex: enable obss scan offload feature flag

We don't perform OBSS scan internally. As we intend to use
corresponding feature in application, we will enable this flag.

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>