[3/4] I/OAT: fix async_tx.callback checking

async_tx.callback should be checked for the first
not the last descriptor in the chain.

Cc: <stable@kernel.org>
Signed-off-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>