ShellPkg: Add check logic for the gateway validity.

ShellPkg: Add check logic for the gateway validity.

if we set a static IP using command
'ifconfig -s eth0 static 192.168.0.121 255.255.255.0 0.0.0.0'
The system says 'Failed to set address.' but using
'ifconfig -l', the static IP can be assigned successfully.
so we need to check the gateway validity before setting manual
address to keep the ifconfig -s command more consistent.

Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Santhapur Naveen <naveens@amiindia.co.in>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>