crypto: tcrypt - fix ghash-generic speed test

ghash is a keyed hash algorithm, thus setkey needs to be called.
Otherwise the following error occurs:
$ modprobe tcrypt mode=318 sec=1
testing speed of async ghash-generic (ghash-generic)
tcrypt: test  0 (   16 byte blocks,   16 bytes per update,   1 updates):
tcrypt: hashing failed ret=-126

Cc: <stable@vger.kernel.org> # 4.6+
Fixes: 0660511c ("crypto: tcrypt - Use ahash")
Tested-by: Franck Lenormand <franck.lenormand@nxp.com>
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>