i2c: core: Use fwnode_for_each_child_node_scoped()

Replace the manual __free(fwnode_handle) iterator declaration with the
new scoped iterator macro for cleaner, less error-prone code.

This eliminates the need for explicit iterator variable declaration with
the cleanup attribute, making the code more consistent with other scoped
iterator usage patterns in the kernel.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>