net: stmmac: remove write-only priv->speed

priv->speed is only ever written to in two locations, but never
read. Therefore, it serves no useful purpose. Remove this unnecessary
struct member.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1tqLJJ-005aQm-Mv@rmk-PC.armlinux.org.uk


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