i2c: uniphier: replace debug() with dev_dbg()

Use dev_dbg() functions.  It will be helpful to prefix log messages
with the corresponding device name when the core framework is ready.

While I am here, I renamed "dev", which was actually private data,
into "priv" because dev->dev looks confusing.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>