NFCT: fix crash in polling mode if used by two stacks

NFCT: fix crash in polling mode if used by two stacks

The polling timer initialisation code was put in the configurator
code. It was then created for all instances. But only first one has
a valid NFCT handle. This was resulting in a crash.
This patch moves the timer initialisation in the constructor which
is called only once on the main NFCT instance.

Signed-off-by: Eric Leblond <eric@regit.org>
Reported-by: Gomathivinayagam Muthuvinayagam <sankarmail@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>