drivers/mmc/tegra2_mmc.c: fix GCC 4.6 warning

Fix:
tegra2_mmc.c: In function 'mmc_send_cmd':
tegra2_mmc.c:230:3: warning: 'mask' may be used uninitialized in this
function [-Wuninitialized]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Doug Anderson <dianders@chromium.org>
Cc: Tom Warren <twarren@nvidia.com>