ASoC: soc-utils: make snd_soc_platform_driver const

Make this const as it is only passed as the 2nd argument to the function
snd_soc_register_platform, which is of type const.
Done using Coccinelle

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>