sh: dma: use __maybe_unused

There is no such thing as labeling a variable as __attribute__((used)).  Since
ts_shift is not referenced in inline assembly, we assume that we're simply
suppressing a warning here if the variable is declared but unreferenced.

Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>