sound: constify snd_device_ops

[Lionel Debroux]

The fields of this struct type don't seem to be used as rvalues, besides
the initialization of struct instances. Let's make const the struct
instances.

Maybe the Documentation/DocBook/writing-an-alsa-driver.tmpl, and
checkpatch.pl should be updated to reflect this change ?

Patch against current Linus tree.

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