flasher: fix assignment of default data_dir
flasher: fix assignment of default data_dir

The default value was being assigned to a global rather than a field in
the argparser result.

Reported-by: Thierry Reding <b75a4d02013b0e68ae2c6bb0dcfd4b7bec282dc9@gmail.com>
Signed-off-by: Stephen Warren <5ef2a23ba3aff51d1cfc8c113c1ec34b608b3b13@nvidia.com>

NVIDIA/tegra-uboot-flasher-scripts