PAN: Avoid reconfiguration of bt-pan interface in NAP role

Issue:
Browsing is not working with remote PANU Device(s).

RCA:
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.

BUG: 29992925
Change-Id: Ib6e0cb91d573649af45ffe25c7095a4bd74a5022