NetworkPkg: Fix Assert issue in iSCSI driver.

NetworkPkg: Fix Assert issue in iSCSI driver.

The bug existed in replacing AsciiStrToUnicodeStr with AsciiStrToUnicodeStrS,
since MacString now is a pointer, the value sizeof(MacString)/sizeof (MacString[0])
is not correct here as the third parameter.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@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>
Reviewed-by: Star Zeng <star.zeng@intel.com>