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

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

 drivers/usb/host/xhci-dbgtty.c:19: warning: Function parameter or member 'port' not described in 'dbc_send_packet'
 drivers/usb/host/xhci-dbgtty.c:19: warning: Function parameter or member 'packet' not described in 'dbc_send_packet'
 drivers/usb/host/xhci-dbgtty.c:19: warning: Function parameter or member 'size' not described in 'dbc_send_packet'

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/20200703174148.2749969-17-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>