libertas: Fix two buffer overflows at parsing bss descriptor

add_ie_rates() copys rates without checking the length
in bss descriptor from remote AP.when victim connects to
remote attacker, this may trigger buffer overflow.
lbs_ibss_join_existing() copys rates without checking the length
in bss descriptor from remote IBSS node.when victim connects to
remote attacker, this may trigger buffer overflow.
Fix them by putting the length check before performing copy.

This fix addresses CVE-2019-14896 and CVE-2019-14897.
This also fix build warning of mixed declarations and code.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Wen Huang <huangwenabc@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
