NetworkPkg UefiPxeBcDxe: Fix build error for lastest VS2015 compiler

NetworkPkg UefiPxeBcDxe: Fix build error for lastest VS2015 compiler

The UefiPxeBcDxe module encounters a build error for IA32 arch using the
latest version of VS2015:

UefiPxeBcDxe.lib(PxeBcBoot.obj) : error LNK2001: unresolved external
symbol __allmul

The cause is line 148 in file NetworkPkg\UefiPxeBcDxe\PxeBcBoot.c. The
third parameter for gBS->SetTimer() function is of type UINT64, so the
multiplication should use the MultU64x32() function now.

Cc: Siyuan Fu <siyuan.fu@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>