ath: Suppress set but unused warnings

The ath driver has a lot of these warnings. It's an older driver, so
just supress these warnings until they can be fixed. They are a mix of
simple dead stores, debubgging output and stuff that would require
careful study to know if its safe to remove the access or not (there are
likely very few of the latter, but if there are any they are latent bugs
that compiler could optimize away). Since I have no ath hardware to test
on anymore, take the conservative approach.

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