pwm: use seq_puts() instead of seq_printf()

For a constant format without additional arguments, use seq_puts()
instead of seq_printf(). Also, the following checkpatch warning is
fixed.

  WARNING: Prefer seq_puts to seq_printf

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>