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>