staging: rtl8723au: collect_bss_info23a(): Eliminate smatch warning

Make length an unsigned int. This should avoid Smatch warning that
'length' can go negative.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>