gpio: tegra: use named constants

In order to make it clear what the parameters to set_config() and
set_direction() mean, and similarly for the return values from the
respective get_*(), define named constants for these values.

Disassembly shows no diff in the generated code, except that the
order of the code in the branches of tegra_gpio_get_function() gets
modified without affecting behaviour.

Suggested-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>