net: stmmac: Add dedicated XPCS cleanup method

Currently the XPCS handler destruction is performed in the
stmmac_mdio_unregister() method. It doesn't look good because the handler
isn't originally created in the corresponding protagonist
stmmac_mdio_unregister(), but in the stmmac_xpcs_setup() function. In
order to have more coherent MDIO and XPCS setup/cleanup procedures,
let's move the DW XPCS destruction to the dedicated stmmac_pcs_clean()
method.

This method will also be used to cleanup PCS hardware using the
pcs_exit() callback that will be introduced to stmmac in a subsequent
patch.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Co-developed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Hariprasad Kelam <hkelam@marvell.com>
Link: https://lore.kernel.org/r/20240513-rzn1-gmac1-v7-2-6acf58b5440d@bootlin.com


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