memory: tegra20-mc: Fix hang in IRQ handler.

In Tegra20 any memory controller interrupt would cause an
infinite loop in the IRQ handler.

Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>