crypto: mediatek - make symbol of_crypto_id static

Fixes the following sparse warning:

drivers/crypto/mediatek/mtk-platform.c:585:27: warning:
 symbol 'of_crypto_id' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>