ppc: Prevent inifnite loop in decrementer auto-reload.

If the DECAR register is set to 0, QEMU tries to reload the decrementer with
zero in an inifinite loop. According to PPC documentation, the decrementer is
triggered on 1->0 transition, so avoid reloading the decrementer if if is
already zero.

The problem does not manifest under Linux, but it is valid to set DECAR to zero
(and may make sense as part of decrementer initialization when interrupts are
disabled).

Signed-off-by: Roman Kapl <rka@sysgo.com>
[dwg: Fixed style nit]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
