sparc64: fix typo in property

There is a typo in a comment that propagated into code:
upa-portis instead of upa-portid

This problem was detected by code inspection.

Fixes: eea98334 ("sparc64: broken %tick frequency on spitfire cpus"
Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reported-by: Steven Sistare <steven.sistare@oracle.com>
Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>