usb: dwc2: host: Rename some fields in struct dwc2_qh

This no-op change just does some renames to simplify a future patch.

1. The "interval" field is renamed to "host_interval" to make it more
   obvious that this interval may be 8 times the interval that the
   device sees (if we're doing split transactions).  A future patch will
   also add the "device_interval" field.
2. The "usecs" field is renamed to "host_us" again to make it more
   obvious that this is the time for the transaction as seen by the
   host.  For split transactions the device may see a much longer
   transaction time.  A future patch will also add "device_us".
3. The "sched_frame" field is renamed to "next_active_frame".  The name
   "sched_frame" kept confusing me because it felt like something more
   permament (the QH's reservation or something).  The name
   "next_active_frame" makes it more obvious that this field is
   constantly changing.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>