scd:openpgp: Allow PIN length of 6 also with a reset code.

* scd/app-openpgp.c (do_change_pin): Fix PIN length check.  Add "R"
flag to the reset code prompt.

When using the reset code it was not possible to set a PIN of length

6. The "R" flags fixes a funny prompt.

• Fixes-commit: efe325ffdf21205b90f888c8f0248bbd4f61404b