r92su: fix panic when adding mic/icv

The network core does not guarantee that every frame has the
needed headroom and tailroom available. So, the driver has
to check whenever there's enough required free room for the
extra data we add.

Reported-by: José Vázquez <ppvazquezfer@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>