pwm: sysfs: Use put_device() instead of kfree()

Never directly free @dev after calling device_register(), even if it
returned an error! Always use put_device() to give up the reference
initialized.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>