ARM: tegra: pcie: 0 -> NULL changes

Fixes:

arch/arm/mach-tegra/pcie.c:465:10: warning: Using plain integer as NULL pointer

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>