xhci: fix even more unsafe memory usage in xhci tracing

Removes static char buffer usage in the following decode functions:
	xhci_decode_ctrl_ctx()
	xhci_decode_slot_context()
	xhci_decode_usbsts()
	xhci_decode_doorbell()
	xhci_decode_ep_context()

Caller must provide a buffer to use.
In tracing use __get_str() as recommended to pass buffer.

Minor changes are needed in other xhci code as these functions are also
used elsewhere

Cc: <stable@vger.kernel.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20210820123503.2605901-3-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>