usb: xhci: Allow context state errors when halting an endpoint

There is a race where an endpoint may halt by itself while we are trying
to halt it, which results in a context state error. See xHCI 4.6.9 which
mentions this case.

This also avoids BUGging when we attempt to stop an endpoint which was
already stopped to begin with, which is probably a bug elsewhere but
not a good reason to crash.

Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Marek Vasut <marex@denx.de>