rsa: Fix two errors in the implementation

1. Failure to set the return code correctly
2. Failure to detect the loop end condition when the value is equal to
the modulus.

Reported-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/rsa/rsa-sign.c b/lib/rsa/rsa-sign.c
index f4d4338..5d9716f 100644
--- a/lib/rsa/rsa-sign.c
+++ b/lib/rsa/rsa-sign.c
@@ -76,6 +76,7 @@
 	rsa = EVP_PKEY_get1_RSA(key);
 	if (!rsa) {
 		rsa_err("Couldn't convert to a RSA style key");
+		ret = -EINVAL;
 		goto err_rsa;
 	}
 	fclose(f);