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

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin