ath5k tracing uses

#define TRACE_INCLUDE_PATH ../../drivers/net/wireless/ath/ath5k

This is problematic because it means that if you build the
backported modules against a full kernel tree the build
uses the original trace.h, not the one from backport, and
this fails (due to the config symbol mangling, but it's more
generally an issue)

Fix this by doing the same all other drivers do.