Minor fixes to ReadSPR and WriteSPR

Nothing special.
What's the purpose of those breaks?
```
		case 0x100: 
		case 0x101:
		case 0x102:
		case 0x103:
		case 0x104:
		case 0x105:
		case 0x106:
		case 0x107: return CPU.USPRG[n - 0x100];
```
is another way of writing:
```
		case 0x100: return CPU.USPRG[0];
		case 0x101: return CPU.USPRG[1];
		case 0x102: return CPU.USPRG[2];
		case 0x103: return CPU.USPRG[3];
		case 0x104: return CPU.USPRG[4];
		case 0x105: return CPU.USPRG[5];
		case 0x106: return CPU.USPRG[6];
		case 0x107: return CPU.USPRG[7];
```