hsl: close socket if we destroy the signalling link
hsl: close socket if we destroy the signalling link

As we do in ipaccess driver, the socket is not useful anymore
if the BSC/BTS decides to close the signalling link.

osmobuntu/libosmo-abis,telenoobie/libosmo-abis,osmocom/libosmo-abis,telenoobie/libosmo-abis,osmobuntu/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis,osmobuntu/libosmo-abis,osmobuntu/libosmo-abis