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

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

 drivers/usb/early/xhci-dbc.c:10: warning: Function parameter or member 'fmt' not described in 'pr_fmt'

Cc: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200706133341.476881-28-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>