NetworkPkg: Fix the issue EfiPxeBcDhcp() may return wrong status.

NetworkPkg: Fix the issue EfiPxeBcDhcp() may return wrong status.

if the instance of the EFI DHCP4 protocol driver is in the Dhcp4Bound status
that is DHCP configuration has completed, so the Dhcp4->Start FUNC in
the PxeBcDhcpDora() will return EFI_ALREADY_STARTED status which lead to
EfiPxeBcDhcp FUNC not in correspondence with UEFI spec.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
[lersek@redhat.com: updated copyright year as requested by Siyuan]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18049 6f19259b-4bc3-4df7-8a09-765794883524