net: phylink: move phylink_pcs_neg_mode() into phylink.c

Move phylink_pcs_neg_mode() from the header file into the .c file since
nothing should be using it.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>