Make sure xHC completes the configure endpoint command and xhci driver
sets the ring pointers correctly before we create the user readable
debugfs file.

In theory there was a small gap where a user could have read the
debugfs file and cause a NULL pointer dereference error as ring
pointer was not yet set, in practise we want this change to simplify
the upcoming streams debugfs support.

Fixes: 02b6fdc2 ("usb: xhci: Add debugfs interface for xHCI driver")
Signed-off-by: NMathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20200918131752.16488-10-mathias.nyman@linux.intel.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>