[MIPS] excite: use __maybe_unused

Replace variable instances of __attribute__((unused)) with
__maybe_unused.

Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Thomas Koeller <thomas.koeller@baslerweb.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>