xhci: Fix null pointer dereference during S4 resume when resetting ep0

xhci: Fix null pointer dereference during S4 resume when resetting ep0

During device enumeration usb core resets endpoint 0 if the max packet
size value differs from the one read from the device descriptor.

usb core will additionally reset endpoint 0 during S4 resume, before
re-enumerating the device, if the device has a reset-resume flag set.

In this case the xhci device representation vdev may be lost due to
xHC restore error and re-initialization during S4 resume.

Make sure slot_id and vdev are valid before trying to re-configure max
packet size during endpoint 0 reset.
max packet size will be re-configured later during re-enumeration.

This fixes commit e34900f46cd6 ("xhci: Reconfigure endpoint 0 max packet
size only during endpoint reset") which is currently in usb-next,
on its way to 6.8

Fixes: e34900f46cd6 ("xhci: Reconfigure endpoint 0 max packet size only during endpoint reset")
Tested-by: Wendy Wang <wendy.wang@intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20231215125707.1732989-2-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>