backlight: pm8941-wled: Fix ptr_ret.cocci warnings

drivers/video/backlight/pm8941-wled.c:404:1-3: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>