net: phylink: disable PCS polling over major configuration

While we are performing a major configuration, there is no point having
the PCS polling timer running. Stop it before we begin preparing for
the configuration change, and restart it only once we've successfully
completed the change.

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>