regulator: lp3971 - remove unnecessary ret value checking in lp3971_i2c_write()

i2c_smbus_write_byte_data() returns zero or negative value,
therefore no need to check if ret is greater than zero or not.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>