PM: sleep: Replace strncmp() with str_has_prefix()

Use str_has_prefix() instead of strncmp() which is less
straightforward in decode_state().

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>