regulator: slg51000: Fix a couple NULL vs IS_ERR() checks

The devm_gpiod_get_from_of_node() function never returns NULL, it
returns error pointers on error.

Fixes: a867bde3 ("regulator: slg51000: add slg51000 regulator driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20190808103335.GD30506@mwanda


Signed-off-by: Mark Brown <broonie@kernel.org>