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

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

if the instance of the DHCP4 protocol driver is in the Dhcp4Bound
status that is DHCP configuration has completed, so the Dhcp4->Start
FUNC in the EfiPxcBcDhcp() 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: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
[lersek@redhat.com: updated copyright year as Siyuan asked]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18050 6f19259b-4bc3-4df7-8a09-765794883524