powerpc: Move checks in pseries_mach_cpu_die()

Rearrange condition checks for better code readability and
	prevention of possible race conditions when
	preferred_offline_state can potentially change during the
	execution of pseries_mach_cpu_die().  The patch will make
	pseries_mach_cpu_die() put cpu in one of the consistent states
	and not hit the run over BUG()

Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>