regulator: rt4831: Fix return value check in rt4831_regulator_probe()

regulator: rt4831: Fix return value check in rt4831_regulator_probe()

In case of error, the function dev_get_regmap() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

Fixes: a73f6dcd3b46 ("regulator: rt4831: Adds support for Richtek RT4831 DSV regulator")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210305034930.3236099-1-weiyongjun1@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>