MdeModulePkg Xhci: Correct description of Timeout param in XhciReg.h

MdeModulePkg Xhci: Correct description of Timeout param in XhciReg.h

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=653

Correct description of Timeout param in XhciReg.h to be matched with
XhciReg.c.

Cc: Alexei Fedorov <Alexei.Fedorov@arm.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>