net: stmmac: platform: provide stmmac_pltfr_init()

Provide a helper wrapper around calling the platform's init() callback.
This allows users to skip checking if the callback exists.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20230623100417.93592-2-brgl@bgdev.pl
Signed-off-by: Jakub Kicinski <kuba@kernel.org>