backlight: pwm_bl: Remove error message upon devm_kzalloc() failure

No need to have a specific OOM message, since there is generic MM out of memory
message in place.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>