Pan: Fix to avoid reconfiguration of bt-pan interface in NAP role

Issue: Browsing is not working  with 2 REMOTE PANU.

RCA: A case, where DUT is in NAP role, bt-pan interface
is configured for NAP role, when 1st remote PANU is
connected. bt-pan interface is being reconfigured, whenever
2nd PANU is connected, which leads bt-pan interface
to go in bad state. BT is able to write Ethernet packets on
bt-pan interface(TUN driver) after reconfiguration, but it's
unable to read Ethernet packets from bt-pan interface(TUN driver).

Fix: If bt-pan interface is already configured in NAP role,
then do not reconfigure bt-pan, when 2nd PANU is connected.

Change-Id: Ib6e0cb91d573649af45ffe25c7095a4bd74a5022