drm/tegra: Fix cast to restricted __le32

Sparse warns about the following cast in the function
falcon_copy_firmware_image() ...

 drivers/gpu/drm/tegra/falcon.c:66:27: warning: cast to restricted __le32

Fix this by casting the firmware data array to __le32 instead of u32.

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