Input: pwm-beeper - switch to using "atomic" PWM API

The "atomic" API allows us to configure PWM period and duty cycle and
enable it in one call.

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