powerpc/powernv: Rate limit opal-elog read failure message

Sometimes we can't read an error log from OPAL, and we print an error
message accordingly. But the OPAL userspace tools seem to like retrying a
lot, in which case we flood the kernel log with a lot of messages.

Change pr_err() to pr_err_ratelimited() to help with this.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201211021140.28402-1-ajd@linux.ibm.com