core/conio.inc: when reading the serial port, drop read of IIR

core/conio.inc: when reading the serial port, drop read of IIR

Drop the read of the IIR when reading the serial port.  First of all,
we weren't protecting AL so we clobbered the actual data; second of
all, this isn't actually necessary as reading RDR will clear the
interrupt condition per the UART spec.

It's worth noting that enabling interrupts here will do bad things if
the interrupts aren't edge-triggered (since we don't actually have an
interrupt routine to clear the interrupt condition); this also means
bad things will happen if the interrupt line is shared.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>