linux config: enable SND CA0132 DSP loading

Since we select everything as a module, snd_hda_codec_ca0132 is built as
well. DSP loading is not enabled by default, but without it the
soundcard produces timeouts within ALSA and does not emit sound.
Explicitly enable the firmware loading to ensure Soundblaster
Z/Zx/ZxR/Recon devices can be used with NixOS.
The patch to enable this by default in the kernel is staged for 5.8.