usb: dwc2: fix the incorrect bitmaps for the ports of multi_tt hub

The dwc2_get_ls_map() use ttport to reference into the
bitmap if we're on a multi_tt hub. But the bitmaps index
from 0 to (hub->maxchild - 1), while the ttport index from
1 to hub->maxchild. This will cause invalid memory access
when the number of ttport is hub->maxchild.

Without this patch, I can easily meet a Kernel panic issue
if connect a low-speed USB mouse with the max port of FE2.1
multi-tt hub (1a40:0201) on rk3288 platform.

Fixes: 9f9f09b0

 ("usb: dwc2: host: Totally redo the microframe scheduler")
Cc: <stable@vger.kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: William Wu <william.wu@rock-chips.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>