Fix lost serial TX interrupts. Report receive overruns.

o Implement receive overrun status.  The FreeBSD uart driver
   relies on this status in it's probe routine to determine the size
   of the FIFO supported.
 o As per the 16550 spec, do not overwrite the RX FIFO on an RX overrun.
 o Do not allow TX or RX FIFO overruns to increment the data valid count
   beyond the size of the FIFO.
 o For reads of the IIR register, only clear the "TX holding register
   emtpy interrupt" if the read reports this interrupt.  This is required
   by the specification and avoids losing TX interrupts when other,
   higher priority interrupts (usually RX) are reported first.

Signed-off-by: Justin T. Gibbs <gibbs@FreeBSD.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
