ASoC: sunxi: make snd_soc_codec_driver structures as const

Declare snd_soc_codec_driver structures as const as they are either
passed as an argument to the function snd_soc_register_codec or stored as
reference in field codec of type sun4i_codec_quirks. Both the fucntion
argument and the codec field are of type const, so declare the
structures with this property as const.

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