crypto: aspeed - fix uninitialized symbol 'idx' warning

'idx' is not initialized if it's not EXP_MODE nor MOD_MODE.
Use "else" instead to fix it.

Signed-off-by: Neal Liu <neal_liu@aspeedtech.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/202302261052.CVFRyq6F-lkp@intel.com/


Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>