net: phylink: remove duplicated linkmode pause resolution

Phylink had two chunks of code virtually the same for resolving the
negotiated pause modes. Factor this down to one function.

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>