Input: pwm-beeper - switch to using managed resources

Use of managed resources (devm) simplifies error handling and tear down
of the driver.

Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Tested-by: David Lechner <david@lechnology.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>