PM / Sleep: Simplify device_suspend_noirq()

Remove a few if () and return statements in device_suspend_noirq()
that aren't really necessary.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>