Netmap: Fix Netmap Polling Issue

Netmap: Fix Netmap Polling Issue

Previously, netmap node switched from interrupt to polling if
worker threads are enabled even if there was no netmap interface
running. Netmap input node consumed CPU cycles because of input
node calls but these cycles were wasted as input node did not have
any packet to process.

This patch fixes issue stated above using more intelligent way of enabling
or disabling polling based on interfaces count. But the pre-requisite
condition for polling is, worker thread(s) should be enabled.

Change-Id: I3a901823c7b040cce3393958981f439b9f290fe5
Signed-off-by: Mohsin KAZMI <sykazmi@cisco.com>