soc/tegra: pmc: Simplify debugfs initialization

debugfs calls should generally not be error-checked to simplify the case
where debugfs is disabled. Since this driver is built-in and has the
sysfs bind/unbind attributes disabled, it cannot be unloaded, so there
is no need to hold onto a reference to the debugfs files that are
created.

We can further simplify this by moving the debugfs file creation to a
later stage to avoid any cleanup we might have to do during error unwind
operations. This is also a little cleaner because the debugfs file
relies on data structures that are created at a later point than when
the file was previously created.

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