kernel - work around ipmi serial port bug

* On our supermicro blade server the ipmi can get confused when the
  host initializes the 16550A and may fail to clear the RXRDY interrupt
  status, resulting in an endless loop.

  This appears to only occur when interrupts are enabled early to support
  kern.alt_break_to_debugger on a serial console.

* Issuing a dummy read of the RXDATA register appears to unstick the ipmi.
  Go figure.