media: igorplugusb: respect DMA coherency

The coherency rules mean that you cannot embed
a buffer inside a descriptor. kmalloc() separately.

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>