PCI UART: better cope with UART being temporarily unavailable

PCI UART: better cope with UART being temporarily unavailable

This happens for example when dom0 disables ioport responses during PCI
subsystem initialisation. If a __ns16550_poll() happens to be scheduled
during that time, Xen hangs. Detect and exit that condition.

Amended ns16550_ioport_invalid function to only check IER register,
which contins 3 reserved (always 0) bits, therefore it's sufficient for
that test.

Signed-off-by: Tomasz Wroblewski <tomasz.wroblewski@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>