MdeModulePkg/UhciPei: Initialize the variable RequestMap

MdeModulePkg/UhciPei: Initialize the variable RequestMap

RequestMap is used but not Initialized.
RequestMap is called by UhciMapUserRequest, in which RequestMap(Map)
is called by IoMmuMap, and is finally called by IoMmu->Map.
We can not assume RequestMap is given an initial value at any step.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>