tegra: i2c: Change driver to use helper function

Now that we have uclass_first_device_drvdata(), use it from the I2C driver
to reduce code duplication.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>