powerpc/rtasd: Don't start event scan if scan rate is zero

powerpc/rtasd: Don't start event scan if scan rate is zero

There appear to be Pegasos systems which have the rtas-event-scan
RTAS tokens, but on which the event scan always fails. They also
have an event-scan-rate property containing 0, which means call
event scan 0 times per minute.

So interpret a scan rate of 0 to mean don't scan at all. This fixes
the problem on the Pegasos machines and makes sense as well.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>