Gekko: Make UReg_MSR's single-argument constructor explicit

Prevents implicit construction of MSR instances from integral values.
This is beneficial, considering MSR values have an intended
representation while a regular magic value doesn't. So make these
conversions required to be explicit.

Source/Core/Core/PowerPC/PowerPC.cpp

@@ -144,7 +144,7 @@ static void ResetRegisters()
   SystemTimers::TimeBaseSet();
 
   // MSR should be 0x40, but we don't emulate BS1, so it would never be turned off :}
-  ppcState.msr = 0;
+  ppcState.msr.Hex = 0;
   rDEC = 0xFFFFFFFF;
   SystemTimers::DecrementerSet();
 }