Input: pwm-beeper - use input_set_capability()

Instead of manipulating capability bits directly, let's use
input_set_capability() API.

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