openocd: src: fix incorrect SPDX tags

openocd: src: fix incorrect SPDX tags

The SPDX tag is aimed at machine handling and it's thus expected
to be placed in the first line in specific format.

Move the SPDX tag to the first line and fix it where needed.

The SPDX tag on files *.c is incorrect, as it should use the C99
single line comment using '//'. But current checkpatch doesn't
allow C99 comments, so keep using standard C comments, by now.

Change-Id: Ie9a05f530009d482a4116eebd147fd7e1ee3d41e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7066
Tested-by: jenkins