usb: host: xhci-dbgcap: File headers are not good candidates for kerneldoc

Demote xhci-dbgcap's file header to a standard comment block.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/host/xhci-dbgcap.c:20: warning: Function parameter or member 'xhci' not described in 'dbc_dma_alloc_coherent'
 drivers/usb/host/xhci-dbgcap.c:20: warning: Function parameter or member 'size' not described in 'dbc_dma_alloc_coherent'
 drivers/usb/host/xhci-dbgcap.c:20: warning: Function parameter or member 'dma_handle' not described in 'dbc_dma_alloc_coherent'
 drivers/usb/host/xhci-dbgcap.c:20: warning: Function parameter or member 'flags' not described in 'dbc_dma_alloc_coherent'

Cc: Mathias Nyman <mathias.nyman@intel.com>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-16-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-dbgcap.c

 // SPDX-License-Identifier: GPL-2.0
-/**
+/*
  * xhci-dbgcap.c - xHCI debug capability support
  *
  * Copyright (C) 2017 Intel Corporation