fix logic of ykp_get_supported_key_length()
fix logic of ykp_get_supported_key_length()

was reporting wrong size for yubico-challenge

dmitriykuptsov/yubikey-personalization,dmitriykuptsov/yubikey-personalization,eworm-de/yubikey-personalization,Yubico/yubikey-personalization-dpkg,dmitriykuptsov/yubikey-personalization,Yubico/yubikey-personalization,Yubico/yubikey-personalization-dpkg,eworm-de/yubikey-personalization,Yubico/yubikey-personalization-dpkg,eworm-de/yubikey-personalization,Yubico/yubikey-personalization