net: stmmac: make the pcs_lynx cleanup sequence specific to dwmac_socfpga

So far, only the dwmac_socfpga variant of stmmac uses PCS Lynx. Use a
dedicated cleanup sequence for dwmac_socfpga instead of using the
generic stmmac one.

Fixes: 5d1f3fe7 ("net: stmmac: dwmac-sogfpga: use the lynx pcs driver")
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>