usb: xhci: Guard all calls to xhci_wait_for_event

xhci_wait_for_event returns NULL on timeout, so the caller always has to
check for that. This addresses immediate explosions in this part
of the code when timeouts happen, but not the root cause for the
timeout.

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