memory: tegra: Fix compilation warnings on 64bit platforms

Fix compilation warning on 64bit platforms caused by implicit promotion
of 32bit signed integer to a 64bit unsigned value which happens after
enabling compile-testing of the EMC drivers.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>