staging: rtl8188eu: prevent an underflow in rtw_check_beacon_data()

The "len" could be as low as -14 so we should check for negatives.

Fixes: 9a7fe54d ("staging: r8188eu: Add source files for new driver - part 1")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>