net: phylink: provide phylink_get_inband_type()

Provide a function to get the type of the inband signalling used for
a PHY interface type. This will be used in the subsequent patch to
address problems with 10G optical modules.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uslws-00000001SP5-1R2R@rmk-PC.armlinux.org.uk


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