leds-lm3530: Fix smatch warnings

Fixes the following smatch warnings:
drivers/leds/leds-lm3530.c:361 lm3530_mode_set() info:
why not propagate 'mode' from lm3530_get_mode_from_str() instead of -22?
drivers/leds/leds-lm3530.c:432 lm3530_probe() info:
why not propagate 'err' from lm3530_init_registers() instead of -19?
drivers/leds/leds-lm3530.c:438 lm3530_probe() info:
why not propagate 'err' from led_classdev_register() instead of -19?

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Shreshtha Kumar SAHU <shreshthakumar.sahu@stericsson.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>