usb: ehci-hcd: Keep async schedule running

Profiling the EHCI driver shows a significant performance problem in
ehci_submit_async(). Specifically, this function keeps enabling and
disabling async schedule back and forth for every single transaction.
However, enabling/disabling the async schedule does not take effect
immediatelly, but instead may take up to 1 mS (8 uFrames) to complete.

This impacts USB storage significantly, esp. since the recent reduction
of maximum transfer size to support more USB storage devices. This in
turn results in sharp increase in the number of ehci_submit_async()
calls. Since one USB storage BBB transfer does three such calls and
the maximum transfer size is 120 kiB, the overhead is 6 mS per 120 kiB,
which is unacceptable.

However, this overhead can be removed simply by keeping the async
schedule running. Specifically, the first transfer starts the async
schedule and then each and every subsequent transfer only adds a new
QH into that schedule, waits until the QH is completed and does NOT
disable the async schedule. The async schedule is stopped only by
shutting down the controller, which must happen before moving out
of U-Boot, otherwise the controller will corrupt memory.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>

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