net: stmmac: platform: provide stmmac_pltfr_exit()

Provide a helper wrapper around calling the platform's exit() 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/20230623100417.93592-4-brgl@bgdev.pl


Signed-off-by: Jakub Kicinski <kuba@kernel.org>