diag: Do not close the server in case for peripheral restart

Currently, apps diag is closing and re-opening the server channel
in case for peripheral restart. In some cases new address update
may not happen for peripheral and peripheral may still send data
to old address. This patch ensures that the server is never
closed.

Change-Id: I4c1740f968e68afe66904f66afc0e4b1bc4fd66d
Signed-off-by: Katish Paran <kparan@codeaurora.org>