i2c: core: Reuse fwnode variable where it makes sense

Reuse fwnode variable where it makes sense. This avoids unneeded
duplication hidden in some macros and unifies the code for different
types of fwnode.

Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>