leds: lp5521: Fix return value

Return the value obtained from i2c_smbus_read_byte_data() instead of
-EIO. Silences the following smatch warning:
drivers/leds/leds-lp5521.c:155 lp5521_read() info:
why not propagate 'ret' from i2c_smbus_read_byte_data() instead of -5?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>