sparc32: fix sparcstation 5 boot

The sparcstation 5 I have available has no MID property for the CPU.
This resulted in a panic when booting a SMP kernel on this box.

The assigned field in cpu_data is never used, so if we fail
to read the MID property then inform user and continue booting.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>