firmware: tegra: Fix error application of sizeof() to pointer

Application of sizeof() to pointer yields the number of bytes of the
pointer, but it should use the length of buffer in the code.

Fixes: 06c2d9a0 ("firmware: tegra: Reduce stack usage")
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>