[POWERPC] Fix hardware IRQ time accounting problem.

The commit fa13a5a1

 (sched: restore
deterministic CPU accounting on powerpc), unconditionally calls
update_process_tick() in system context.  In the deterministic
accounting case this is the correct thing to do.  However, in the
non-deterministic accounting case we need to not do this, since doing
this results in the time accounted as hardware irq time being
artificially elevated.

Also this collapses 2 consecutive '#ifdef CONFIG_VIRT_CPU_ACCOUNTING'
checks in time.h into one for neatness.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>