soc/tegra: pmc: Fix incorrect DPD request

Reading the DPD_REQ & DPD2_REQ registers returns the previous requests.
If we sets the current request bit with the returned value, then other
pads will be turned on or off unexpectedly.

Signed-off-by: Vince Hsu <vinceh@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>