scd: Support changing key attribute back to RSA.

* scd/app-openpgp.c (change_rsa_keyattr): Try usual RSA.

In the OpenPGP card specification, there are multiple options to
support RSA (having P and Q or not, etc.), and it is implementation
dependent. Since GnuPG doesn't have knowledge which card
implementation support which option and there is no way (yet) for card
to express itself which key attributes are supported, we haven't
supported key attribute change back to RSA. But, many card
implementation uses P and Q, try this option. If other cases,
factory-reset would be easier option.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>