rsa: cosmetic: rename pad_len to key_len

checksum_algo's pad_len field isn't actually used to store the length of
the padding but the total length of the RSA key (msg_len + pad_len)

Signed-off-by: Andrew Duda <aduda@meraki.com>
Signed-off-by: aduda <aduda@meraki.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/image.h

@@ -1070,7 +1070,7 @@ struct image_region {
 struct checksum_algo {
 	const char *name;
 	const int checksum_len;
-	const int pad_len;
+	const int key_len;
 #if IMAGE_ENABLE_SIGN
 	const EVP_MD *(*calculate_sign)(void);
 #endif