regulator: tps6586x: Use dev_err rather than dev_warn for error message

tps6586x_regulator_set_slew_rate() returns -EINVAL when having slew rate
settings for other than SM0/1, thus use dev_err rather than dev_warn.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>