media: imon_raw: respect DMA coherency

No buffer can be embedded inside a descriptor, not even a simple be64.
Use a separate kmalloc()

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>