dpdk: auto threads assign one too many threads

dpdk: auto threads assign one too many threads

Configuration option `threads: auto` in DPDK's interface node
overassigns available threads to the interface.
Commit 4dfd44d3 changed the signedness of the remaining threads counter,
which caused surpass of the counter initialization.
The if-clause is switched to first initialize and then use the counter.

Ticket: 7798