NetworkPkg:Fix Http boot download issue.

When http boot download the second time without return
out of the boot manager, the DHCP process will start twice
with the same Boot file uri and print the information twice
which we not expected. This is caused by wrong logic
of handling the device path of the boot file when loading it.

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: Fu Siyuan <siyuan.fu@intel.com>