UART: Make the 8250's MCR return a deterministic value.

This change makes the 8250 device return the value it has for the MCR when
read instead of leaving the packet data unmodified/uninitialized. The value
the UART has for the MCR may not be right, but that's a seperate issue that
apparently hasn't caused any problems to date.

src/dev/uart8250.cc

@@ -161,6 +161,7 @@ Uart8250::read(PacketPtr pkt)
             pkt->set(LCR);
             break;
         case 0x4: // Modem Control Register (MCR)
+            pkt->set(MCR);
             break;
         case 0x5: // Line Status Register (LSR)
             uint8_t lsr;