Update GATT channel state to closing during disconnection

This change ensures that the GATT channel state is
moved to CLOSING state while GATT disconnect is
requested by the host, specifically when the GATT
state is OPEN.
This is needed to prevent any furthur GATT operations,
once GATT disconnect is issued.

Change-Id: I8511caa477881de4f4eaf18b84f23530e1958fe9