power: reset: qcom-pon: reg write mask depends on pon generation

Instead of hardcode the mask, it should be depends
on which generation of pon it was.

Signed-off-by: Kejia Hu <kejia.hu@codethink.co.uk>
Fixes: fce5430f ("reset: qcom-pon: Add support for gen2 pon")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>