net: stmmac: sti: convert to devm_stmmac_pltfr_probe()

Convert sti to use the generic devm_stmmac_pltfr_probe() which will
call plat_dat->init()/plat_dat->exit() as appropriate, thus
simplifying the code.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1u4jMj-000rCM-31@rmk-PC.armlinux.org.uk


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