ASoC: Remove 'const' from the device_node pointers

As Russell King's explained it, there should not be pointers to
struct device_node:

"struct device_node is a ref-counted structure.  That means if you
 store a reference to it, you should "get" it, and you should "put"
 it once you've done.  The act of "put"ing the pointed-to structure
 involves writing to that structure, so it is totally unappropriate
 to store a device_node structure as a const pointer.  It forces you
 to have to cast it back to a non-const pointer at various points
 in time to use various OF function calls."

[This isn't quite the application here, we're not geting or putting the
pointer though we did add some other users who call non-const OF
functions -- broonie]

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>