net: stmmac: unexport stmmac_pltfr_init/exit()

These functions are only used within the compilation unit they're defined
in so there's no reason to export them.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240619140119.26777-1-brgl@bgdev.pl


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