xHCI: replace dev_dbg() with xhci_dbg()

dev_dbg() is used to print ordinary transfer messages in xhci-ring.c.
System log messages will be flushed if CONFIG_USB_DEBUG is set. Replace the
dev_dbg() with xhci_dbg().

Signed-off-by: Andiry Xu <andiry.xu@amd.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>