net: stmmac: don't rely on lynx_pcs presence to check for a PHY

When initializing attached PHYs, there are some cases where we don't expect
any PHY to be connected. The logic uses conditions based on various local
PCS configuration, but also calls-in phylink_expects_phy() via
stmmac_init_phy(), which is enough to ensure we don't try to initialize a
PHY when using a Lynx PCS, as long as we have the phy_interface set to a
802.3z mode and are using inband negociation.

Drop the lynx check, making the stmmac generic code more pcs_lynx-agnostic.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
[rgantois: commit log]
Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240326-rxc_bugfix-v6-3-24a74e5c761f@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>