net: phylink: ensure manual flow control is selected appropriately

Split the application of manually controlled flow control modes from
phylink_resolve_flow(), so that we can use alternative providers of
flow control resolution.

We also want to clear the MLO_PAUSE_AN flag when autoneg is disabled,
since flow control can't be negotiated in this circumstance.

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