spapr_events: drop bogus cell from "interrupt-ranges" property

According to LoPAPR 1.1 B.6.12, the "/event-sources" node has an "interrupt-
ranges" property, the format of which is described in B.6.9.1.2 as follows:

“interrupt-ranges”
 Standard property name that defines the interrupt number(s) and range(s)
 handled by this unit.

 prop-encoded-array: List of (int-number, range) specifications.

 Int-number is encoded as with encode-int.
 Range is encoded as with encode-int.

 The first entry in this list shall contain the int-number associated with
 the first “reg” property entry. The int-num-ber is the value representing
 the interrupt source as would appear in the PowerPC External Interrupt
 Architecture XISR. The range shall be the number of sequential interrupt
 numbers which this unit can generate.

There's no such thing as a cell count at the end of the array, like the
one introduced by commit ffbb1705 in QEMU 2.8. It doesn't seem it had
any impact on existing guests and I couldn't find any related workaround
in linux. So, let's just drop the bogus lines.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>