gpio: tegra: Iterate over the correct number of banks

When Tegra30 support was added to the Tegra GPIO driver, a few places
which iterated over all banks were not converted to use the variable
tegra_gpio_bank_count rather than hard-coding the bank count. Fix this.

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