i2c: tegra: Only display error messages if DMA setup fails

Commit 86c92b99 ("i2c: tegra: Add DMA support") added DMA support
to the Tegra I2C driver for Tegra devices that support the APB DMA
controller. One side-effect of this change is that even for Tegra
devices that do not have an APB DMA controller and hence, cannot
support DMA tranfers for I2C transactions, the following error messages
are still displayed ...

 ERR KERN tegra-i2c 31c0000.i2c: cannot use DMA: -19
 ERR KERN tegra-i2c 31c0000.i2c: falling back to PIO

There is no point displaying the above messages for devices that do not
have an APB DMA controller and so fix this by returning from the
tegra_i2c_init_dma() function if 'has_apb_dma' is not true.

Furthermore, if CONFIG_TEGRA20_APB_DMA is not set, then rather than
printing an error message, print an debug message as for whatever reason
this could be intentional.

Fixes: 86c92b99 ("i2c: tegra: Add DMA support")
Signed-off-by: Jonathan Hunter <jonathanh@nvidia.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>