ShellPkg/Ping: fix loss of first packet

ShellPkg/Ping: fix loss of first packet

Issue:
Reply for first ping packet was getting dropped.

Cause:
Sometimes reply message comes even before trasmit
function returns, hence missing 1st reply

Fix:
Prepare the TxList before calling Transmit function.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Meenakshi Aggarwal <meenakshi.aggarwal@nxp.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>