net: phylink: split out PHY validation from phylink_bringup_phy()

When bringing up a PHY, we need to work out which ethtool link modes it
should support and advertise. Clause 22 PHYs operate in a single
interface mode, which can be easily dealt with. However, clause 45 PHYs
tend to switch interface mode depending on the media. We need more
flexible validation at this point, so this patch splits out that code
in preparation to changing it.

Tested-by: Luo Jie <quic_luoj@quicinc.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1r6VIQ-00DDM9-LK@rmk-PC.armlinux.org.uk


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