pldmtool: Close socket connection

With pldm commands like getPDR, pldmtool opens new socket for each PDR.
In case we have hundred number of PDRs, pldmtool open hundred number of
socket connection to server. It causes mctp-demux (role as server) reach
limit number of clients can handle.

So pldmtool should close socket connection to server after completely
using it.

Signed-off-by: Dung Cao <dung@os.amperecomputing.com>
Change-Id: I66fb00cd2d51f131c8901a8964552bcb2fc1de13