ehci: Fix interrupts stopping when Interrupt Threshold Control is 8

ehci: Fix interrupts stopping when Interrupt Threshold Control is 8

If Interrupt Threshold Control is 8 or a multiple of 8, then
s->usbsts_frindex can become exactly 0x4000, at which point
(s->usbsts_frindex > s->frindex) will never become true, as
s->usbsts_frindex will not be lowered / reset in this case.

This patch fixes this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit ffa1f2e088eb7e3d57f2fc35f21e7bdb23e592c5)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>