mwifiex: remove redundant initialization for bss_descriptor

Initialization of bss_descriptor is unnecessary as the entire
structure will be overwritten by a memcpy followed by.

Initialize disable_11ac flag properly by setting it to true in
mwifiex_fill_new_bss_desc().

Reported-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>