serial/8250_pci: Clear FIFOs for Intel ME Serial Over Lan device on BI

When using Serial Over Lan (SOL) over the virtual serial port in a Intel
management engine (ME) device, on device reset the serial FIFOs need to
be cleared to keep the FIFO indexes in-sync between the host and the
engine.

On a reset the serial device assertes BI, so using that as a cue FIFOs
are cleared.  So for this purpose a new handle_break callback has been
added.  One other problem is that the serial registers might temporarily
go to 0 on reset of this device.  So instead of using the IER register
read, if 0 returned use the ier value in uart_8250_port. This is hidden
under a custom serial_in.

Cc: Nhan H Mai <nhan.h.mai@intel.com>
Signed-off-by: Sudhakar Mamillapalli <sudhakar@fb.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>