regulators: max8998.c: use dev_err() instead of printk()

Fixes the following checkpatch warning:

WARNING: Prefer netdev_err(netdev, ... then dev_err(dev, ... then pr_err(...  to printk(KERN_ERR ...

Signed-off-by: Thiago Farina <tfarina@chromium.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>