leds: lp3944: Fix return value

Return the value obtained from i2c_smbus_read_byte_data() instead of
-EINVAL. Silences the following smatch warning:
drivers/leds/leds-lp3944.c:89 lp3944_reg_read() info:
why not propagate 'tmp' from i2c_smbus_read_byte_data() instead of -22?

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