drivers: ethernet: w5500: make `ethernet_api` as `const`

This change marks `w5500_api_funcs`, an instance of
the `ethernet_api`, as `const`.

By using `const`, we ensure immutability, leading to usage of only
`.rodata` and a reduction in the `.data` area.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>