staging/goldfish/goldfish_audio: Fix annotations

dmam_alloc_coherent does not return a __iomem pointer.
here is its prototype:

void * dmam_alloc_coherent(struct device *dev, size_t size,
			   dma_addr_t *dma_handle, gfp_t gfp)

This fixes these sparse warnings:

drivers/staging/goldfish/goldfish_audio.c:134:43: warning: incorrect
type in argument 2 (different address spaces)
drivers/staging/goldfish/goldfish_audio.c:134:43:    expected void const
*from
drivers/staging/goldfish/goldfish_audio.c:134:43:    got char [noderef]
<asn:2>*read_buffer
drivers/staging/goldfish/goldfish_audio.c:167:36: warning: incorrect
type in argument 1 (different address spaces)
drivers/staging/goldfish/goldfish_audio.c:167:36:    expected void *to
drivers/staging/goldfish/goldfish_audio.c:167:36:    got char [noderef]
<asn:2>*[assigned] kbuf
drivers/staging/goldfish/goldfish_audio.c:296:27: warning: incorrect
type in assignment (different address spaces)
drivers/staging/goldfish/goldfish_audio.c:296:27:    expected char
[noderef] <asn:2>*buffer_virt
drivers/staging/goldfish/goldfish_audio.c:296:27:    got void *

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>