iio: adc: ti_am335x_adc: Replace license text with SPDX tag

Drop the text license and replace it with an equivalent SPDX license tag
identifier which also matches the MODULE_LICENSE("GPL").
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-40-miquel.raynal@bootlin.com

drivers/iio/adc/ti_am335x_adc.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * TI ADC MFD driver
  *
  * Copyright (C) 2012 Texas Instruments Incorporated - https://www.ti.com/
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License as
- * published by the Free Software Foundation version 2.
- *
- * This program is distributed "as is" WITHOUT ANY WARRANTY of any
- * kind, whether express or implied; without even the implied warranty
- * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
  */
 
 #include <linux/kernel.h>