net: prestera: update PCS driver to use neg_mode

Update prestera's embedded PCS driver to use neg_mode rather than the
mode argument. As there is no pcs_link_up() method, this only affects
the pcs_config() method.

Acked-by: Elad Nachman <enachman@marvell.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qA8EO-00EaG3-TR@rmk-PC.armlinux.org.uk


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