linux-amazon-biscuit: remove dummy mmap from pcm audio driver

Disables mmap capability on the biscuit PCM audio driver, fixing a kernel panic when starting pulseaudio.

I'm not entirely sure why MediaTek thought passing a fake page address was a good idea for their audio driver, but this patch at least stops the kernel from exploding trying to map a dummy address. Once this and some pulse + ucm configs are merged, we should have (semi) working audio on biscuit.

amazon-{karnak,douglas} use the same audio driver and apparently suffer from similar kernel panic issues when starting pulse. May be worth an owner of one of these devices trying this patch if it fixes it there too. amazon-ford also has a similar audio driver that does this fake page address trickery.