hw/ppc: Always store the decrementer value

hw/ppc: Always store the decrementer value

When writing a value to the decrementer that raises an exception, the
irq is raised, but the value is not stored so the store doesn't appear
to have changed the register when it is read again.

Always store the write value to the register.

Fixes: e81a982aa53 ("PPC: Clean up DECR implementation")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>