crypto: ixp4xx - Fix false lastlen uninitialised warning

This patch fixes a false positive uninitialised variable warning
in aead_perform by moving the source processing in front of the
destination processing, thus ensuring that the initialisation of
lastlen is always visible to gcc.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>