drivers: usb: dwc_otg: fix reference passing when checking bandwidth

The pointer (struct usb_host_endpoint *)->hcpriv should contain a
reference to dwc_otg_qh_t if the driver has already seen a URB submitted
to this endpoint.

It then checks whether the qh exists and is already in a schedule in
order to decide whether to allocate periodic bandwidth or not. Passing a
pointer to an offset inside of struct usb_host_endpoint instead of just
the pointer means it dereferences bogus addresses.

Rationalise (delete) a variable while we're at it.

See https://github.com/raspberrypi/linux/issues/5189



Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>