net: stmmac: dwmac-generic: use stmmac_pltfr_exit()

Shrink the code in dwmac-generic by using the new stmmac_pltfr_exit()
helper.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20230623100417.93592-5-brgl@bgdev.pl


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