drivers/regulator: fix a missing check of return value

If palmas_smps_read() fails, we should not use the read data in "reg"
which may contain random value. The fix inserts a check for the return
value of palmas_smps_read(): If it fails, we return the error code
upstream and stop using "reg".

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Mark Brown <broonie@kernel.org>