ShellPkg: Fix ping IPv6 stack usage mode failure issue

ShellPkg: Fix ping IPv6 stack usage mode failure issue

Fix ping IPv6 stack usage mode failure issue and also
update its the help info.
Verified command:
*ping -?
*Ping -_ip6 -_s 2000:bbbb::12 2000:bbbb::8

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17940 6f19259b-4bc3-4df7-8a09-765794883524