PM / Freezer: fix return value of freezable_schedule_timeout_killable()

...it should return the return code from schedule_timeout_killable(),
not the one from freezer_count().

All of the current callers ignore the return code so the bug is
harmless but it's worth fixing.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>