gpio: tegra186: Rename flow variable to type

The IRQ core code refers to the interrupt type by that name, whereas the
term flow is almost never used. Some GPIO controllers use the term
flow_type, but it is most consistent to just go with the IRQ core
terminology.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>