Input: pwm-beeper - fix - scheduling while atomic

Pwm config may sleep so defer it using a worker.

On a Freescale i.MX53 based board we ran into "BUG: scheduling while
atomic" because input_inject_event locks interrupts, but
imx_pwm_config_v2 sleeps.

Tested on Freescale i.MX53 SoC with 4.6.0.

Signed-off-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>