PM / sleep: Refine diagnostic messages in enter_state()

Some of the system suspend diagnostic messages related to
suspend-to-idle refer to it as "freeze sleep" or "freeze state"
while the others say "suspend-to-idle".  To reduce the possible
confusion that may result from that, refine the former either to
say "suspend to idle" too or to make it clearer that what is printed
is a state string written to /sys/power/state ("mem", "standby",
or "freeze").

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>