usb: class: usbtmc: File headers are not good candidates for kerneldoc

Demote usbtmc's file header to a standard comment block.

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

 drivers/usb/class/usbtmc.c:11: warning: Function parameter or member 'fmt' not described in 'pr_fmt'
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200702144625.2533530-30-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/usbtmc.c

 // SPDX-License-Identifier: GPL-2.0+
-/**
+/*
  * drivers/usb/class/usbtmc.c - USB Test & Measurement class driver
  *
  * Copyright (C) 2007 Stefan Kopp, Gechingen, Germany