soc/tegra: fuse: Add FUSE clock check in tegra_fuse_readl()

tegra_fuse_readl() can be called from drivers at any time. If this API
is called before tegra_fuse_probe(), we end up enabling the clock before
it is registered. Add a check for the FUSE clock in tegra_fuse_readl()
and propagate any errors.

Signed-off-by: Nagarjuna Kristam <nkristam@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>