scd: Fix hard-coded constant for RSA auth.

* scd/app-openpgp.c (do_auth): Allow larger data for RSA-4096.

OpenPGPcard specification says that it will be rejected by the card
when it's larger. We have been the check on host side too, but it was
written when it only had a support for RSA-2048.

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