net: pcs: xpcs: clean up reading clause 73 link partner advertisement

Read the clause 73 link partner advertisement in a loop and then
translate to the ethtool modes.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>