crypto: ecdsa - Replace ndigits with nbits where precision is needed

Replace the usage of ndigits with nbits where precise space calculations
are needed, such as in ecdsa_max_size where the length of a coordinate is
determined.
Tested-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ecdsa.c

@@ -266,7 +266,7 @@ static unsigned int ecdsa_max_size(struct crypto_akcipher *tfm)
 {
 	struct ecc_ctx *ctx = akcipher_tfm_ctx(tfm);
 
-	return ctx->pub_key.ndigits << ECC_DIGITS_TO_BYTES_SHIFT;
+	return DIV_ROUND_UP(ctx->curve->nbits, 8);
 }
 
 static int ecdsa_nist_p384_init_tfm(struct crypto_akcipher *tfm)