PM / Sleep: Make pm_op() and pm_noirq_op() return callback pointers

Make the pm_op() and pm_noirq_op() functions return pointers to
appropriate callbacks instead of executing those callbacks and
returning their results.

This change is required for a subsequent modification that will
execute the corresponding driver callback if the subsystem
callback returned by either pm_op(), or pm_noirq_op() is NULL.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>