regulator: tps65218: Add the missing of_node assignment in probe

Add the missing of_node assignment in probe.

Fixes: 90e7d526 (regulator: tps65218: Add Regulator driver for TPS65218 PMIC)
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: <stable@vger.kernel.org> # v3.15