crypto: mediatek - move mtk_aes_find_dev() to the right place

Move mtk_aes_find_dev() to right functions as nobody uses the
'cryp' under current flows.

We can also avoid duplicate checks here and there in this way.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Vic Wu <vic.wu@mediatek.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>