ASoC: ak4458: Modify SPDX license format

Modify SPDX comment style to match the rules. Since the original
code had a GPL-2.0 license and SPDX is compatible with GPL-2.0+,
we also modified the MODULE_LICENSE.

Signed-off-by: Cosmin-Gabriel Samoila <cosmin.samoila@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>