crypto: arm/crc32 - avoid warning when compiling with Clang

The table id (second) argument to MODULE_DEVICE_TABLE is often
referenced otherwise. This is not the case for CPU features. This
leads to a warning when building the kernel with Clang:
  arch/arm/crypto/crc32-ce-glue.c:239:33: warning: variable
    'crc32_cpu_feature' is not needed and will not be emitted
    [-Wunneeded-internal-declaration]
  static const struct cpu_feature crc32_cpu_feature[] = {
                                  ^

Avoid warnings by using __maybe_unused, similar to commit 1f318a8b
("modules: mark __inittest/__exittest as __maybe_unused").

Fixes: 2a9faf8b ("crypto: arm/crc32 - enable module autoloading based on CPU feature bits")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>