examples/ioat: fix failure check for ioat dequeue

examples/ioat: fix failure check for ioat dequeue

The nb_dq return value from the ioat dequeue is negative in failure
cases, however the variable was an unsigned int, causing the condition
where nb_dq <= 0 to never be true. This is now cast to a signed int,
which will successfully reflect the -1 value to be used in this
conditional check.

Coverity issue: 350342
Coverity issue: 350349
Fixes: 92c981637ffc ("examples/ioat: handle failure case for ioat dequeue")
Cc: stable@dpdk.org
Signed-off-by: Ciara Power <ciara.power@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>