Input: pwm-beeper - fix race when suspending

Usually userspace sends SND_BELL and SND_TONE events, and by the time
pwm_beeper_suspend() runs userpsace is already frozen, but theoretically
in-kernel users may send these events too, and that may cause
pwm_beeper_event() scheduling another work after we canceled it.

Let's introduce a "suspended" flag and check it in pwm_beeper_event() to
avoid this race.

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