i2c: tegra: Clean up variable names

Rename "ret" variables to "err" in order to make code a bit more
expressive, emphasizing that the returned value is an error code.
Same vice versa, where appropriate.

Rename variable "reg" to "val" in order to better reflect the actual
usage of the variable in the code and to make naming consistent with
the rest of the code.

Use briefer names for a few members of the tegra_i2c_dev structure in
order to improve readability of the code.

All dev/&pdev->dev are replaced with i2c_dev->dev in order to have uniform
code style across the driver.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>