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

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

Root Cause:
When the 1st remote PANU is connected, the bt-pan interface gets
configured. As soon as 2nd PANU is connected, the bt-pan interface
goes into a bad state. The second device is able to write Ethernet
packets on the bt-pan interface (TUN driver) after reconfiguration,
but it is unable to read Ethernet packets from the interface.

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

Bug: 29992925
Change-Id: Ib6e0cb91d573649af45ffe25c7095a4bd74a5022