mwifiex: fix use-after-free in beacon_ie processing

beacon_ie buffer is allocated in mwifiex_fill_new_bss_desc()
and the buffer pointer is saved in bss_desc->beacon_buf.
beacon_ie is freed before the function returns. However,
bss_desc->beacon_buf is still being accessed afterwards.

Fix it by freeing beacon_ie (bss_desc->beacon_buf) in
caller's scope.

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: John W. Linville <linville@tuxdriver.com>