staging: rtl8723au: rtw_check_beacon_data23a(): Make len unsigned int

Making len unsigned int fixes a warning caused by smatch getting
confused that len could be 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>