lib/dpif-netdev: Clean-up pmd thread signaling.

It could be possible that the thread misses a signal when it reads the
change_seq again after reload.  Also, the counter has no dependent
data, so the memory model for the atomic read can be relaxed.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>