net: stmmac: explain the phylink_speed_down() call in stmmac_release()

The call to phylink_speed_down() looks odd on the face of it. Add a
comment to explain why this call is there. phylink_speed_up() is
always called in __stmmac_open(), and already has a comment.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1umsfV-008vKv-1O@rmk-PC.armlinux.org.uk


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