crypto: mediatek - fix uninitialized value of gctx->textlen

Add a pre-computed text length to avoid uninitialized value in the check.

Fixes: e4727066 ("crypto: mediatek - Add empty messages check in GCM mode")
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>