ssh/key-func.c: Use `_scm_from_ssh_key'

* ssh/key-func.c (guile_ssh_string_to_public_key)
  (guile_ssh_private_key_from_file)
  (guile_ssh_public_key_from_private_key)
  (guile_ssh_public_key_from_file): Use `_scm_from_ssh_key'.