staging: exfat - fix SPDX tags..

The copyright notices as I got them said "GPLv2 or later", which I
screwed up when putting in the SPDX tags.  Fix them to match the
license I got the code under.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>

Link: https://lore.kernel.org/r/122590.1568853488@turing-police


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>