net: phylink: allow mac_select_pcs() to remove a PCS

phylink has historically not permitted a PCS to be removed. An attempt
to permit this with phylink_set_pcs() resulted in comments indicating
that there was no need for this. This behaviour has been propagated
forward to the mac_select_pcs() approach as it was believed from these
comments that changing this would be NAK'd.

However, with mac_select_pcs(), it takes more code and thus complexity
to maintain this behaviour, which can - and in this case has - resulted
in a bug. If mac_select_pcs() returns NULL for a particular interface
type, but there is already a PCS in-use, then we skip the pcs_validate()
method, but continue using the old PCS. Also, it wouldn't be expected
behaviour by implementers of mac_select_pcs().

Allow this by removing this old unnecessary restriction.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>