Input: snvs_pwrkey - fix returned value check of syscon_regmap_lookup_by_phandle()

On error syscon_regmap_lookup_by_phandle() returns ERR_PTR() value,
which makes a check for NULL invalid and may lead to oops on error
path.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>